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D3 v3 cumulative time-series line chart showing gaps in data

Option B

cumulative line chart, with gaps in data shown as dotted lines

PROS:

missing data clearly differentiated.

CONS:

data points with missing data the month before and after are 'hidden' (e.g. May 2014).
difficult to notice spikes or trends in views/downloads.

Features:

gaps in data shown with dotted line
mini 'brush' chart shows the context of the data in focus in the main chart
x-axis zooming and panning is limited to the range of available data
y-axis resizes automatically to match the maximum y-value of the data in focus
buttons zoom focus to an interval equal to one year (if sufficient data), one month, or the range of the data
text at top changes to indicate the range of data in focus
when no data is available, displays text instead
sorts data by date

<!DOCTYPE html>
<meta charset="utf-8">
<style>

/******************************************
* Metric Modal Chart (Views, Downloads, Citations)
********************************************/

/* When there is no data ...*/

#metric-modal .metric-chart text {
	fill: #565656;
	font-size: 9px;
	font-family: Helvetica, Arial, "sans serif";
}

#metric-modal .metric-chart text.no-data {
	font-size: 16px;
	font-weight: 100;
	fill:#d0d0d0;
}

#metric-modal .metric-chart rect.no-data {
	fill: #f5f5f5;
}

/* When there is data ...*/

/* CB: padding to display better on bl.ocks.org */
#metric-modal {
	padding: 64px;
}

#metric-modal .metric-chart rect.plot-background{
	fill: white;
}

/* complete data line */
#metric-modal .metric-chart path.line {
	fill: none;
	stroke: #00AA8D; /* default, changed in each theme */
	stroke-width: 1.5px;
	clip-path: url(#clip);
}

/* complete data area */
#metric-modal .metric-chart path.area {
	fill: #00AA8D; /* CB default, changed in each theme */
	opacity: 0.6;
	clip-path: url(#clip);
	cursor: move; /* fallback if grab cursor is unsupported */
	cursor: grab;
}

/*missing data line */
#metric-modal .metric-chart path.line_missing {
	fill: none;
	stroke: #00AA8D; /* default, changed in each theme */
	stroke-width: 1.5px;
	stroke-dasharray: 1.5px 2px;
	clip-path: url(#clip);
}
/*missing data area */
#metric-modal .metric-chart path.area_missing {
	fill: #00AA8D; /* CB default, changed in each theme */
	opacity: 0.15;
	clip-path: url(#clip);
	cursor: move; /* fallback if grab cursor is unsupported */
	cursor: grab;
}

#metric-modal .metric-chart path.area_missing:active,
#metric-modal .metric-chart path.area:active {
	cursor: move; /* fallback if grab cursor is unsupported */
	cursor: grabbing;
}

#metric-modal .metric-chart .axis {
	shape-rendering: crispEdges;
}

#metric-modal .metric-chart .x.axis .domain{
	display:none;
}

#metric-modal .metric-chart .x.axis line {
	stroke: white;
	opacity: 0.4;
}

#metric-modal .metric-chart .context .x.axis line {
	display: none;
}

#metric-modal .metric-chart .y.axis .domain{
	display: none;
}

#metric-modal .metric-chart .y.axis.title{
	font-size: 13px;
	font-weight: 100;
}

#metric-modal .metric-chart .y.axis line {
	stroke: #565656;
	stroke-dasharray: 2,2;
	stroke-opacity: 0.3;
}

#metric-modal .metric-chart .brush .extent {
  fill-opacity: .07;
  shape-rendering: crispEdges;
  clip-path: url(#clip);
}

#metric-modal .metric-chart rect.pane {
	cursor: move; /* fallback if grab cursor is unsupported */
	cursor: grab;
	fill: white;
	pointer-events: all;
}

#metric-modal .metric-chart rect.pane:active {
	cursor: move; /* fallback if grab cursor is unsupported */
	cursor: grabbing;
}

/* brush handles  */
#metric-modal .metric-chart .resize .handle {
	fill: #555;
}

#metric-modal .metric-chart .resize .handle-mini {
	fill: white;
    stroke-width: 1px;
    stroke: #555;
}

#metric-modal .metric-chart .scale_button {
	cursor: pointer;
}

#metric-modal .metric-chart .scale_button rect {
	fill: #eaeaea;
}
#metric-modal .metric-chart .scale_button:hover text {
	fill: #417DD6;
	transition: all 0.1s cubic-bezier(.25,.8,.25,1);
}

#metric-modal .metric-chart text#displayDates  {
	font-weight: bold;
}

}
</style>
<body>
<div id="metric-modal"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js"></script>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

// example data
var metricName   = "views";
var metricCount  = [1, 3, 1, 2, 1, 1, 1, 1, 2, 2, 3, 1, 2, 1, 4, 3, 2, 1, 1, 1, 1, 1, 4, 2, 1, 2, 8, 2, 1, 4, 2, 4, 1, 3, 1, 2, 1, 1, 3, 1, 1, 5, 1, 1, 4];
var metricMonths = ["2018-06", "2013-04", "2015-11", "2012-10", "2014-09", "2014-02", "2016-02", "2016-04", "2016-06", "2014-12", "2013-07", "2017-01", "2015-10", "2012-12", "2013-05", "2018-04", "2015-06", "2017-03", "2014-08",
                    "2017-07", "2013-02", "2012-07", "2016-03", "2017-06", "2018-07", "2014-10", "2013-01", "2013-10", "2017-11", "2014-05", "2012-11", "2015-01", "2018-03", "2015-12", "2015-08", "2016-08", "2014-11", "2014-01",
                    "2013-06", "2012-08", "2015-09", "2016-07", "2013-03", "2012-09", "2016-05"];
var optwidth        = 600;
var optheight       = 370;

/*
* ========================================================================
*  Prepare data
* ========================================================================
*/

// to find missing data: get a sequence of all months between the min and max dates in the data.

// change dates to milliseconds
metricMonths.forEach(function(part, index, theArray) {
  theArray[index] = d3.time.format("%Y-%m").parse(part).getTime();
});

// get a list of all months in the range of data
var monthArray = d3.time.scale()
                .domain(d3.extent(metricMonths))
                .ticks(d3.time.months, 1);

// check if there is data for each month in monthArray, if so append count, otherwise append null
var dataset = [];
var running_total = 0;
for(var i=0; i<monthArray.length; i++){
	var n = metricMonths.indexOf(monthArray[i].getTime());
	if (n>-1) {
		var running_total = running_total + metricCount[n];
		var data_point = true;
	} else {
		var running_total = running_total;
		var data_point = false;
	}
	dataset.push({count: running_total, month: monthArray[i], data_point: data_point});
}

// a dataset without the null values is also needed to draw the missing data lines/areas
var dataset_no_null = dataset;

/*
* ========================================================================
*  sizing
* ========================================================================
*/

/* === Focus chart === */

var margin	= {top: 20, right: 30, bottom: 100, left: 20},
    width	= optwidth - margin.left - margin.right,
    height	= optheight - margin.top - margin.bottom;

/* === Context chart === */

var margin_context = {top: 320, right: 30, bottom: 20, left: 20},
    height_context = optheight - margin_context.top - margin_context.bottom;

/*
* ========================================================================
*  x and y coordinates
* ========================================================================
*/

// the date range of available data:
var dataXrange = d3.extent(dataset, function(d) { return d.month; });
var dataYrange = [0, d3.max(dataset, function(d) { return d.count; })];

// maximum date range allowed to display
var mindate = dataXrange[0],  // use the range of the data
    maxdate = dataXrange[1];

var DateFormat	  =  d3.time.format("%b %Y");

/* === Focus Chart === */

var x = d3.time.scale()
	.range([0, (width)])
    .domain(dataXrange);

var y = d3.scale.linear()
	.range([height, 0])
    .domain(dataYrange);

var xAxis = d3.svg.axis()
	.scale(x)
    .orient("bottom")
		.tickSize(-(height))
    .ticks(customTickFunction)
    .tickFormat(dynamicDateFormat);

var yAxis = d3.svg.axis()
	.scale(y)
    .ticks(4)
    .tickSize(-(width))
	.orient("right");

/* === Context Chart === */

var x2 = d3.time.scale()
    .range([0, width])
    .domain([mindate, maxdate]);

var y2 = d3.scale.linear()
	.range([height_context, 0])
    .domain(y.domain());

var xAxis_context = d3.svg.axis()
    .scale(x2)
    .orient("bottom")
    .ticks(customTickFunction)
    .tickFormat(dynamicDateFormat);

/*
* ========================================================================
*  Plotted line and area variables
* ========================================================================
*/

/* === Focus Chart === */

var line = d3.svg.line()
	.defined(function(d) { return d.data_point == true; })
	.x(function(d) { return x(d.month); })
	.y(function(d) { return y(d.count); });

var area = d3.svg.area()
	.defined(line.defined())
	.x(function(d) { return x(d.month); })
	.y0((height))
	.y1(function(d) { return y(d.count); });

var line_missing = d3.svg.line()
	.x(function(d) { return x(d.month); })
	.y(function(d) { return y(d.count); });

var area_missing = d3.svg.area()
	.x(function(d) { return x(d.month); })
	.y0((height))
	.y1(function(d) { return y(d.count); });

/* === Context Chart === */

var line_context = d3.svg.line()
	.defined(function(d) { return d.data_point !== false; })
    .x(function(d) { return x2(d.month); })
    .y(function(d) { return y2(d.count); });

var area_context = d3.svg.area()
	.defined(line_context.defined())
    .x(function(d) { return x2(d.month); })
    .y0((height_context))
    .y1(function(d) { return y2(d.count); });

var line_context_missing = d3.svg.line()
	.x(function(d) { return x2(d.month); })
	.y(function(d) { return y2(d.count); });

var area_context_missing = d3.svg.area()
	.x(function(d) { return x2(d.month); })
	.y0((height_context))
	.y1(function(d) { return y2(d.count); });

/*
* ========================================================================
*  Variables for brushing and zooming behaviour
* ========================================================================
*/

var brush = d3.svg.brush()
    .x(x2)
    .on("brush", brushed)
    .on("brushend", brushend);

var zoom = d3.behavior.zoom()
    .on("zoom", draw)
    .on("zoomend", brushend);

/*
* ========================================================================
*  Define the SVG area ("vis") and append all the layers
* ========================================================================
*/

// === the main components === //

var vis = d3.select("#metric-modal").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
  	.attr("class", "metric-chart");// CB -- "line-chart" -- CB //

vis.append("defs").append("clipPath")
    .attr("id", "clip")
    .append("rect")
    .attr("width", width)
    .attr("height", height);
    // clipPath is used to keep line and area from moving outside of plot area when user zooms/scrolls/brushes

var rect = vis.append("svg:rect")
    .attr("class", "pane")
    .attr("width", width)
    .attr("height", height)
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

var context = vis.append("g")
    .attr("class", "context")
    .attr("transform", "translate(" + margin_context.left + "," + margin_context.top + ")");

var focus = vis.append("g")
    .attr("class", "focus")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

rect.call(zoom)
	.call(draw);

// === current date range text & zoom buttons === //

var display_range_group = vis.append("g")
    .attr("id", "buttons_group")
    .attr("transform", "translate(" + 0 + ","+ 0 +")");

var expl_text = display_range_group.append("text")
    .text("Showing data from: ")
    .style("text-anchor", "start")
    .attr("transform", "translate(" + 0 + ","+ 10 +")");

display_range_group.append("text")
    .attr("id", "displayDates")
    .text(DateFormat(dataXrange[0]) + " - " + DateFormat(dataXrange[1]))
    .style("text-anchor", "start")
    .attr("transform", "translate(" + 82 + ","+ 10 +")");

var expl_text = display_range_group.append("text")
    .text("Zoom to: ")
    .style("text-anchor", "start")
    .attr("transform", "translate(" + 180 + ","+ 10 +")");

// === the zooming/scaling buttons === //

var button_width = 40;
var button_height = 14;

// don't show year button if < 1 year of data
var dateRange  = dataXrange[1] - dataXrange[0],
    ms_in_year = 31540000000;

if (dateRange < ms_in_year)   {
    var button_data =["month","data"];
} else {
    var button_data =["year","month","data"];
};

var button = display_range_group.selectAll("g")
    .data(button_data)
    .enter().append("g")
    .attr("class", "scale_button")
    .attr("transform", function(d, i) { return "translate(" + (220 + i*button_width + i*10) + ",0)"; })
    .on("click", scaleDate);

button.append("rect")
    .attr("width", button_width)
    .attr("height", button_height)
    .attr("rx", 1)
    .attr("ry", 1);

button.append("text")
    .attr("dy", (button_height/2 + 3))
    .attr("dx", button_width/2)
    .style("text-anchor", "middle")
    .text(function(d) { return d; });

/* === focus chart === */

focus.append("g")
    .attr("class", "y axis")
    .call(yAxis)
   	.attr("transform", "translate(" + (width) + ", 0)");

// missing data area
focus.append("path")
    .datum(dataset_no_null)
    .attr("class", "area_missing")
    .attr("d", area_missing)
	.call(zoom)
	.call(draw);

// complete data area
focus.append("path")
    .datum(dataset)
    .attr("class", "area")
    .attr("d", area)
	.call(zoom)
	.call(draw);

// x-axis
focus.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(xAxis);

// missing data line
focus.append("path")
    .datum(dataset_no_null)
    .attr("class", "line_missing")
    .attr("d", line_missing);

// complete data line
focus.append("path")
    .datum(dataset)
    .attr("class", "line")
    .attr("d", line);

/* === context chart === */

context.append("path")
    .datum(dataset_no_null)
    .attr("class", "area_missing")
    .attr("d", area_context_missing);

context.append("path")
    .datum(dataset)
    .attr("class", "area")
    .attr("d", area_context);

context.append("path")
    .datum(dataset_no_null)
    .attr("class", "line_missing")
    .attr("d", line_context_missing);

context.append("path")
    .datum(dataset)
    .attr("class", "line")
    .attr("d", line_context);

context.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height_context + ")")
    .call(xAxis_context);

/* === brush (part of context chart)  === */

var brushg = context.append("g")
    .attr("class", "x brush")
    .call(brush);

brushg.selectAll(".extent")
   .attr("y", -6)
   .attr("height", height_context + 8);
   // .extent is the actual window/rectangle showing what's in focus

brushg.selectAll(".resize")
    .append("rect")
    .attr("class", "handle")
    .attr("transform", "translate(0," +  -3 + ")")
    .attr('rx', 2)
	.attr('ry', 2)
    .attr("height", height_context + 6)
    .attr("width", 3);

brushg.selectAll(".resize")
    .append("rect")
    .attr("class", "handle-mini")
    .attr("transform", "translate(-2,8)")
    .attr('rx', 3)
    .attr('ry', 3)
    .attr("height", (height_context/2))
    .attr("width", 7);
    // .resize are the handles on either size
    // of the 'window' (each is made of a set of rectangles)

/* === y axis title === */

vis.append("text")
    .attr("class", "y axis title")
    .text("Monthly " + this.metricName)
    .attr("x", (-(height/2)))
    .attr("y", 0)
    .attr("dy", "1em")
    .attr("transform", "rotate(-90)")
    .style("text-anchor", "middle");

// allows zooming before any brush action
zoom.x(x);

/*
* ========================================================================
*  Functions
* ========================================================================
*/

// === tick/date formatting functions ===
// from: https://stackoverflow.com/questions/20010864/d3-axis-labels-become-too-fine-grained-when-zoomed-in

function timeFormat(formats) {
  return function(date) {
    var i = formats.length - 1, f = formats[i];
    while (!f[1](date)) f = formats[--i];
    return f[0](date);
  };
};

function customTickFunction(t0, t1, dt)  {
    var labelSize = 42; //
    var maxTotalLabels = Math.floor(width / labelSize);

function step(date, offset)
    {
        date.setMonth(date.getMonth() + offset);
    }

var time = d3.time.month.ceil(t0), times = [], monthFactors = [1,3,4,12];

while (time < t1) times.push(new Date(+time)), step(time, 1);
    var timesCopy = times;
    var i;
    for(i=0 ; times.length > maxTotalLabels ; i++)
        times = _.filter(timesCopy, function(d){
            return (d.getMonth()) % monthFactors[i] == 0;
        });

return times;
};

// === brush and zoom functions ===

function brushed() {
    x.domain(brush.empty() ? x2.domain() : brush.extent());
	common_behaviour();
    // Reset zoom scale's domain
    zoom.x(x);
    updateDisplayDates();
    setYdomain();

}

function draw() {
    setYdomain();
    common_behaviour();
    // Force changing brush range
    brush.extent(x.domain());
    vis.select(".brush").call(brush);
    // and update the text showing range of dates.
    updateDisplayDates();
}

function common_behaviour() {
	focus.select(".area").attr("d", area);
    focus.select(".line").attr("d", line);
	focus.select(".area_missing").attr("d", area_missing);
    focus.select(".line_missing").attr("d", line_missing);
	focus.select(".x.axis").call(xAxis);
}

function brushend() {
// when brush stops moving:

// check whether chart was scrolled out of bounds and fix,
    var b = brush.extent();
    var out_of_bounds = brush.extent().some(function(e) { return e < mindate | e > maxdate; });
    if (out_of_bounds){ b = moveInBounds(b) };

};

function updateDisplayDates() {

var b = brush.extent();
    // update the text that shows the range of displayed dates
    var localBrushDateStart = (brush.empty()) ? DateFormat(dataXrange[0]) : DateFormat(b[0]),
        localBrushDateEnd   = (brush.empty()) ? DateFormat(dataXrange[1]) : DateFormat(b[1]);

// Update start and end dates in upper right-hand corner
    d3.select("#displayDates")
        .text(localBrushDateStart == localBrushDateEnd ? localBrushDateStart : localBrushDateStart + " - " + localBrushDateEnd);
};

function moveInBounds(b) {
// move back to boundaries if user pans outside min and max date.

var ms_in_year = 31536000000,
        brush_start_new,
        brush_end_new;

if       (b[0] < mindate)   { brush_start_new = mindate; }
    else if  (b[0] > maxdate)   { brush_start_new = new Date(maxdate.getTime() - ms_in_year); }
    else                        { brush_start_new = b[0]; };

if       (b[1] > maxdate)   { brush_end_new = maxdate; }
    else if  (b[1] < mindate)   { brush_end_new = new Date(mindate.getTime() + ms_in_year); }
    else                        { brush_end_new = b[1]; };

brush.extent([brush_start_new, brush_end_new]);

brush(d3.select(".brush").transition());
    brushed();
    draw();

return(brush.extent())
};

function setYdomain(){
// this function dynamically changes the y-axis to fit the data in focus

// get the min and max date in focus
    var xleft = new Date(x.domain()[0]);
    var xright = new Date(x.domain()[1]);

// a function that finds the nearest point to the right of a point
    var bisectDate = d3.bisector(function(d) { return d.month; }).right;

// get the y value of the line at the left edge of view port:
    var iL = bisectDate(dataset_no_null, xleft);

if (dataset_no_null[iL] !== undefined && dataset_no_null[iL-1] !== undefined) {

var left_dateBefore = dataset_no_null[iL-1].month,
            left_dateAfter = dataset_no_null[iL].month;

var intfun = d3.interpolateNumber(dataset_no_null[iL-1].count, dataset_no_null[iL].count);
        var yleft = intfun((xleft-left_dateBefore)/(left_dateAfter-left_dateBefore));
    } else {
        var yleft = 0;
    }

// get the x value of the line at the right edge of view port:
    var iR = bisectDate(dataset_no_null, xright);

if (dataset_no_null[iR] !== undefined && dataset_no_null[iR-1] !== undefined) {

var right_dateBefore = dataset_no_null[iR-1].month,
            right_dateAfter = dataset_no_null[iR].month;

var intfun = d3.interpolateNumber(dataset_no_null[iR-1].count, dataset_no_null[iR].count);
        var yright = intfun((xright-right_dateBefore)/(right_dateAfter-right_dateBefore));
    } else {
        var yright = 0;
    }

// get the y values of all the actual data points that are in view
    var dataSubset = dataset.filter(function(d){ return d.month >= xleft && d.month <= xright; });
    var countSubset = [];
    dataSubset.map(function(d) {countSubset.push(d.count);});

// add the edge values of the line to the array of counts in view, get the max y;
    countSubset.push(yleft);
    countSubset.push(yright);
    var ymax_new = d3.max(countSubset);

if(ymax_new == 0){
        ymax_new = dataYrange[1];
    }

// reset and redraw the yaxis
    y.domain([0, ymax_new*1.05]);
    focus.select(".y.axis").call(yAxis);

};

function scaleDate(d,i) {
// action for buttons that scale focus to certain time interval

var b = brush.extent(),
        interval_ms,
        brush_end_new,
        brush_start_new;

if      (d == "year")   { interval_ms = 31536000000}
    else if (d == "month")  { interval_ms = 2592000000 };

if ( d == "year" | d == "month" )  {

if((maxdate.getTime() - b[1].getTime()) < interval_ms){
        // if brush is too far to the right that increasing the right-hand brush boundary would make the chart go out of bounds....
            brush_start_new = new Date(maxdate.getTime() - interval_ms); // ...then decrease the left-hand brush boundary...
            brush_end_new = maxdate; //...and set the right-hand brush boundary to the maxiumum limit.
        } else {
        // otherwise, increase the right-hand brush boundary.
            brush_start_new = b[0];
            brush_end_new = new Date(b[0].getTime() + interval_ms);
        };

} else if ( d == "data")  {
        brush_start_new = dataXrange[0];
        brush_end_new = dataXrange[1]
    } else {
        brush_start_new = b[0];
        brush_end_new = b[1];
    };

brush.extent([brush_start_new, brush_end_new]);

// now draw the brush to match our extent
    brush(d3.select(".brush").transition());
    // now fire the brushstart, brushmove, and brushend events
    brush.event(d3.select(".brush").transition());
};

</script>
</body>

https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js

https://d3js.org/d3.v3.min.js