d3 library - how to set the legend values

Question

I'm trying to understand how the legend works with the d3 library.

Here is an example of what I want to do :

http://bl.ocks.org/tjdecke/5558084

I want to be able to fix the legend for example if the value is :

between 0 -> 20 = display green color

between 20 -> 50 = display pink color

between 50 -> 100 = display grey color

My values are correct, it's just that the legend change according to my values.

Sometime it can be :

between 0 -> 2 = green

between 20 -> 6 = pink

between 50 -> 10 = grey

Or :

between 0 -> 10 = green

between 20 -> 15 = pink

between 50 -> 75 = grey

I need a fix legend.

But I don't know where to set this.

Here is the code :

<div id="chart"></div>

    <script type="text/javascript">
      var margin = { top: 50, right: 0, bottom: 100, left: 30 },
          width = 960 - margin.left - margin.right,
          height = 430 - margin.top - margin.bottom,
          gridSize = Math.floor(width / 24),
          legendElementWidth = gridSize*2,
          buckets = 9,
          colors = ["#ffffd9","#edf8b1","#c7e9b4","#7fcdbb","#41b6c4","#1d91c0","#225ea8","#253494","#081d58"], // alternatively colorbrewer.YlGnBu[9]
          days = ["Mo", "Tu", "We", "Th", "Fr", "Sa", "Su"],
          times = ["1a", "2a", "3a", "4a", "5a", "6a", "7a", "8a", "9a", "10a", "11a", "12a", "1p", "2p", "3p", "4p", "5p", "6p", "7p", "8p", "9p", "10p", "11p", "12p"];


      d3.tsv("data.tsv",
        function(d) {
          return {
            day: +d.day,
            hour: +d.hour,
            value: +d.value
          };
        },
        function(error, data) {
          var colorScale = d3.scale.quantile()
              .domain([0, buckets - 1, d3.max(data, function (d) { return d.value; })])
              .range(colors);

          var svg = d3.select("#chart").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 + ")");

          var dayLabels = svg.selectAll(".dayLabel")
              .data(days)
              .enter().append("text")
                .text(function (d) { return d; })
                .attr("x", 0)
                .attr("y", function (d, i) { return i * gridSize; })
                .style("text-anchor", "end")
                .attr("transform", "translate(-6," + gridSize / 1.5 + ")")
                .attr("class", function (d, i) { return ((i >= 0 && i <= 4) ? "dayLabel mono axis axis-workweek" : "dayLabel mono axis"); });

          var timeLabels = svg.selectAll(".timeLabel")
              .data(times)
              .enter().append("text")
                .text(function(d) { return d; })
                .attr("x", function(d, i) { return i * gridSize; })
                .attr("y", 0)
                .style("text-anchor", "middle")
                .attr("transform", "translate(" + gridSize / 2 + ", -6)")
                .attr("class", function(d, i) { return ((i >= 7 && i <= 16) ? "timeLabel mono axis axis-worktime" : "timeLabel mono axis"); });

          var heatMap = svg.selectAll(".hour")
              .data(data)
              .enter().append("rect")
              .attr("x", function(d) { return (d.hour - 1) * gridSize; })
              .attr("y", function(d) { return (d.day - 1) * gridSize; })
              .attr("rx", 4)
              .attr("ry", 4)
              .attr("class", "hour bordered")
              .attr("width", gridSize)
              .attr("height", gridSize)
              .style("fill", colors[0]);

          heatMap.transition().duration(1000)
              .style("fill", function(d) { return colorScale(d.value); });

          heatMap.append("title").text(function(d) { return d.value; });

          var legend = svg.selectAll(".legend")
              .data([0].concat(colorScale.quantiles()), function(d) { return d; })
              .enter().append("g")
              .attr("class", "legend");

          legend.append("rect")
            .attr("x", function(d, i) { return legendElementWidth * i; })
            .attr("y", height)
            .attr("width", legendElementWidth)
            .attr("height", gridSize / 2)
            .style("fill", function(d, i) { return colors[i]; });

          legend.append("text")
            .attr("class", "mono")
            .text(function(d) { return "≥ " + Math.round(d); })
            .attr("x", function(d, i) { return legendElementWidth * i; })
            .attr("y", height + gridSize);
      });
    </script>

Thank you for your help !

Answer 1

If I got your question correctly, you need d3.scale.threshold instead of d3.scale.quantile here.

Say, you need to map the values to colors the following way:

0 - < 3 - color[0]
3 - < 10 - color[1]
10 - < 30 - color[2]
>= 30 - color[3]

Update your code the following way:

// init the scale
var colorScale = d3.scale.threshold()
    .domain([0, 3, 10, 30])
    .range([0].concat(colors));

Use this scale the same way you use it in the chart at the moment. And for the legend use

colorScale.domain()

instead of

[0].concat(colorScale.quantiles()

See the demo