Focus+Context via Brushing

Thank you to mbostock for his block: Focus+Context via Brushing

This was forked and adapted from that starting point.

<!DOCTYPE html>
<meta charset="utf-8">
<link href='https://fonts.googleapis.com/css?family=Raleway:400,700' rel='stylesheet' type='text/css'>
<style>

svg {
  font-family: 'Raleway', sans-serif;
  font-size: 16px;
}

.area {
  fill: none;
  stroke: steelblue;
  stroke-width: 1px;
  clip-path: url(#clip);
}

.axis path,
.axis line {
  fill: none;
  stroke: #000;
  shape-rendering: crispEdges;
}

.tick {
  font-size: 12px;
}

.brush .extent {
  stroke: #fff;
  fill-opacity: .125;
  shape-rendering: crispEdges;
}

.chartTitle {
  font-size: 22px;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script>

/*
var title = {
  'x': 'Date',
  'y': 'C02: parts per million',
  'chart': 'C02 Concentration measured at Mauna Loa: 1974 to 2014'
}

var file = "c02Daily.csv";
*/

var title = {
  'x': 'Date',
  'y': 'Number of Babies Born',
  'chart': 'Number of Babies in the United States: 1969 to 1988'
}

var file = "dailyUSBirths.csv";

var min = 'zero';

var totalHeight = 700,
    bottomChartHeight = 200,
    margin = {top: 30, right: 10, bottom: bottomChartHeight + 30, left: 130},
    margin2 = {top: totalHeight - bottomChartHeight, right: margin.right, bottom: 60, left: margin.left},
    width = 960 - margin.left - margin.right,
    height = totalHeight - margin.top - margin.bottom,
    height2 = totalHeight - margin2.top - margin2.bottom;

var parseDate = d3.time.format("%Y-%m-%d").parse;

var x = d3.time.scale().range([0, width]),
    x2 = d3.time.scale().range([0, width]),
    y = d3.scale.linear().range([height, 0]),
    y2 = d3.scale.linear().range([height2, 0]);

var xAxis = d3.svg.axis().scale(x).orient("bottom"),
    xAxis2 = d3.svg.axis().scale(x2).orient("bottom"),
    yAxis = d3.svg.axis().scale(y).orient("left");

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

var area = d3.svg.line()
    .interpolate("monotone")
    .x(function(d) { return x(d.date); })
    //.y0(height)
    .y(function(d) { return y(d.price); });

var area2 = d3.svg.line()
    .interpolate("monotone")
    .x(function(d) { return x2(d.date); })
    .y(function(d) { return y2(d.price); });

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom);

svg.append("defs").append("clipPath")
    .attr("id", "clip")
  .append("rect")
    .attr("width", width)
    .attr("height", height);

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

var context = svg.append("g")
    .attr("class", "context")
    .attr("transform", "translate(" + margin2.left + "," + margin2.top + ")");

d3.csv(file, type, function(error, data) {
  x.domain(d3.extent(data.map(function(d) { return d.date; })));
  y.domain(
    [ min == 'zero' ? 0 :  d3.min(data.map(function(d) { return d.price; })) * .95,
    1.05 * d3.max(data.map(function(d) { return d.price; }))
    ]);
  x2.domain(x.domain());
  y2.domain(y.domain());

focus.append("path")
      .datum(data)
      .attr("class", "area")
      .attr("d", area);

focus.append("text")
    .attr("x", width / 2)
    .attr("y", -10)
    .attr("dy", ".71em")
    .style("text-anchor", "middle")
    .attr("class", 'chartTitle')
    .text(title.chart);

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

focus.append("g")
      .attr("class", "y axis")
      .call(yAxis)
    .append("text")
    .attr("transform", "rotate(-90)")
    .attr("x", -(totalHeight - bottomChartHeight)/2)
    .attr("y", -90)
    .attr("dy", ".71em")
    .style("text-anchor", "middle")
    .text(title.y);

context.append("path")
      .datum(data)
      .attr("class", "area")
      .attr("d", area2);

context.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height2 + ")")
      .call(xAxis2)
  .append("text")
    .attr("x", width / 2)
    .attr("y", 40)
    .attr("dy", ".71em")
    .style("text-anchor", "middle")
    .text(title.x);

context.append("g")
      .attr("class", "x brush")
      .call(brush)
    .selectAll("rect")
      .attr("y", -6)
      .attr("height", height2 + 7);
});

function brushed() {
  x.domain(brush.empty() ? x2.domain() : brush.extent());
  focus.select(".area").attr("d", area);
  focus.select(".x.axis").call(xAxis);
}

function type(d) {
  d.date = parseDate(d.date);
  d.price = +d.value;
  return d;
}

</script>

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