Brush Snapping II

This brush snaps to day boundaries. As the user brushes, the brush fires brush events, allowing a listener to adjust the brush extent. Compare this approach to using brush transitions on brushend.

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

.axis text {
  font: 11px sans-serif;
}

.axis path {
  display: none;
}

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

.grid-background {
  fill: #ddd;
}

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

.grid .minor.tick line {
  stroke-opacity: .5;
}

.brush .extent {
  stroke: #000;
  fill-opacity: .125;
  shape-rendering: crispEdges;
}

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

var margin = {top: 200, right: 40, bottom: 200, left: 40},
    width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.time.scale()
    .domain([new Date(2013, 2, 1), new Date(2013, 2, 15) - 1])
    .range([0, width]);

var brush = d3.svg.brush()
    .x(x)
    .extent([new Date(2013, 2, 2), new Date(2013, 2, 3)])
    .on("brush", brushed);

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

svg.append("rect")
    .attr("class", "grid-background")
    .attr("width", width)
    .attr("height", height);

svg.append("g")
    .attr("class", "x grid")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.svg.axis()
        .scale(x)
        .orient("bottom")
        .ticks(d3.time.hours, 12)
        .tickSize(-height)
        .tickFormat(""))
  .selectAll(".tick")
    .classed("minor", function(d) { return d.getHours(); });

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.svg.axis()
      .scale(x)
      .orient("bottom")
      .ticks(d3.time.days)
      .tickPadding(0))
  .selectAll("text")
    .attr("x", 6)
    .style("text-anchor", null);

var gBrush = svg.append("g")
    .attr("class", "brush")
    .call(brush);

gBrush.selectAll("rect")
    .attr("height", height);

function brushed() {
  var extent0 = brush.extent(),
      extent1;

// if dragging, preserve the width of the extent
  if (d3.event.mode === "move") {
    var d0 = d3.time.day.round(extent0[0]),
        d1 = d3.time.day.offset(d0, Math.round((extent0[1] - extent0[0]) / 864e5));
    extent1 = [d0, d1];
  }

// otherwise, if resizing, round both dates
  else {
    extent1 = extent0.map(d3.time.day.round);

// if empty when rounded, use floor & ceil instead
    if (extent1[0] >= extent1[1]) {
      extent1[0] = d3.time.day.floor(extent0[0]);
      extent1[1] = d3.time.day.ceil(extent0[1]);
    }
  }

d3.select(this).call(brush.extent(extent1));
}

</script>

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