Small multiple histograms

Small multiple histograms, applies d3.histogram on nested data.

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body {
      font-family:"avenir next", Arial, sans-serif;
      font-size: 12px;
      color: #696969;
      margin: 0;
    }

.axis path, .axis line {
      stroke: darkgrey;
    }
  
    .hist.label {
      font-size: 10px;
    }
  </style>
</head>

var parseDate = d3.timeParse("%d/%m/%Y");
var formatDateIntoYear = d3.timeFormat("%Y");

// x scale for time
var x = d3.scaleTime()
    .range([0, width])
    .clamp(true);

// y scale for histogram
var y = d3.scaleLinear()
    .range([height, 0]);

// load data
d3.csv("data.csv", prepare, function(data) {

x.domain(d3.extent(data, function(d) { return d.date; }));

// set parameters for histogram
  var histogram = d3.histogram()
      .value(function(d) { return d.date; })
      .domain(x.domain())
      .thresholds(x.ticks(d3.timeYear));

// nest data by region
  var nest = d3.nest()
      .key(function(d) { return d.region; })
      .entries(data);

// apply histogram generator to each region's values
  var regions = nest.map(function(d) { 
    return {
      key: d.key,
      values: histogram(d.values)  
    }
  });

// calculate local y-max for each region
  var localMax = regions.map(function(d) {
    return {
      region: d.key,
      max: d3.max(d.values, function(s) {
        return s.length;
      })
    }
  })
  
  // find global max
  y.domain([0, d3.max(localMax, function(d) { return d.max; })])

// for each region, set up a svg with axis and label
  var svg = d3.select("#vis").selectAll("svg")
      .data(regions)
      .enter()
      .append("svg")
      .attr("width", width + margin.left + margin.right)
      .attr("height", height + margin.top + margin.bottom);

svg.append("text")
      .attr("class", "region label")
      .attr("x", margin.left)
      .attr("y", margin.top/2)
      .attr("font-size", "1.2em")
      .text(function(d) { return d.key; })

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

hist.append("g")
      .attr("class", "axis")
      .attr("transform", "translate(0" + "," + height + ")")
      .call(d3.axisBottom(x)
        .tickValues(x.domain())
        .tickFormat(formatDateIntoYear)
      );

// draw histogram bars
  var bars = hist.selectAll(".bar")
      .data(function(d) { return d.values; })
      .enter()
      .append("g")
      .attr("class", "bar")
      .attr("transform", function(s) {
        return "translate(" + x(s.x0) + "," + y(s.length) + ")";
      });

bars.append("rect")
      .attr("class", "bar")
      .attr("x", 1)
      .attr("width", function(s) { return x(s.x1) - x(s.x0); })
      .attr("height", function(s) { return height - y(s.length); })
      .attr("fill", "#c9c9c9");

bars.append("text")
      .attr("class", "hist label")
      .attr("dy", ".75em")
      .attr("y", -12)
      .attr("x", function(s) { return (x(s.x1) - x(s.x0))/2; })
      .attr("text-anchor", "middle")
      .text(function(s) { 
        if (s.length > 0) {
          return s.length;   
        }
      })
      .style("fill", "charcoal");
});

function prepare(d) {
  d.date = parseDate(d.date);
  return d;
}

</script>
</body>

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