Brush Snapping

This brush snaps to day boundaries. On release, the brush fires an end event, allowing a listener to modify the brush selection. Using brush.move to initiate a transition, the brush smoothly interpolates from the original selection to the rounded selection. Compare this approach to using immediate snapping while brushing.

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

.axis--grid .domain {
  fill: #ddd;
  stroke: none;
}

.axis--x .domain,
.axis--grid .tick line {
  stroke: #fff;
}

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

</style>
<body>
<script src="https://d3js.org/d3.v4.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.scaleTime()
    .domain([new Date(2013, 7, 1), new Date(2013, 7, 15) - 1])
    .rangeRound([0, width]);

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("g")
    .attr("class", "axis axis--grid")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x)
        .ticks(d3.timeHour, 12)
        .tickSize(-height)
        .tickFormat(function() { return null; }))
  .selectAll(".tick")
    .classed("tick--minor", function(d) { return d.getHours(); });

svg.append("g")
    .attr("class", "axis axis--x")
    .attr("transform", "translate(0," + height + ")")
    .call(d3.axisBottom(x)
        .ticks(d3.timeDay)
        .tickPadding(0))
    .attr("text-anchor", null)
  .selectAll("text")
    .attr("x", 6);

svg.append("g")
    .attr("class", "brush")
    .call(d3.brushX()
        .extent([[0, 0], [width, height]])
        .on("end", brushended));

function brushended() {
  if (!d3.event.sourceEvent) return; // Only transition after input.
  if (!d3.event.selection) return; // Ignore empty selections.
  var d0 = d3.event.selection.map(x.invert),
      d1 = d0.map(d3.timeDay.round);

// If empty when rounded, use floor & ceil instead.
  if (d1[0] >= d1[1]) {
    d1[0] = d3.timeDay.floor(d0[0]);
    d1[1] = d3.timeDay.offset(d1[0]);
  }

d3.select(this).transition().call(d3.event.target.move, d1.map(x));
}

</script>

https://d3js.org/d3.v4.min.js