[D3] 7. Quantitative Scales

# Quantitative Scales

    var colorScale = d3.scale.quantile()
            .domain([d3.max(dataset) / 4, d3.max(dataset) / 2 , 3*d3.max(dataset) / 4, d3.max(dataset)])
            .range(["#9c9ede","#6b6ecf","#5254a3", "#393b79"]);


So the code divide the value into 4 range, if the value smaller than 1/4 max value, then it use #9c9ede color;
from 1/4 - 1/2: #6b6ecf
...
from 3/4 - 1: #393b79

 var q = d3.scale.quantize().domain([0, 1]).range(['a', 'b', 'c']);
 //q(0.3) === 'a', q(0.4) === 'b', q(0.6) === 'b', q(0.7) ==='c';
 //q.invertExtent('a') returns [0, 0.3333333333333333]



### use:

.attr('fill', colorScale);

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <script src="../bower_components/underscore/underscore-min.js"></script>
    <script src="../ventor/d3.min.js"></script>
    <style type="text/css">

        body
        {
            padding-top: 50px;
            padding-left: 100px;

        }

        #chartArea {
            width: 400px;
            height: 300px;
            background-color: #CCC;
        }

        .bar
        {
            display: inline-block;
            width: 20px;
            height: 75px; /* Gets overriden by D3-assigned height below */
            margin-right: 2px;
           /* fill: teal; *//* SVG doesn't have background prop, use fill instead*/
            z-index:99;
        }

    </style>
</head>
<body>
<section id="chartArea"></section>
<script>
    var dataset = _.map(_.range(30), function(num) {
        return Math.random() * 50;
    }), //reandom generate 15 data from 1 to 50
            w = 400, h = 300;
    var svg = d3.select('#chartArea').append('svg')
            .attr('width', w)
            .attr('height', h); //svg deosn't need 'px'

    var yScale = d3.scale.linear()
            .domain([0, d3.max(dataset) * 1.1]) //d3.max(dataset), set the max val of database
            .range([0, h]);

    var xScale = d3.scale.ordinal()
            .domain(dataset)
            .rangeBands([0,w],0.3, 0.1);

//    var colorScale = d3.scale.category20c();
    var colorScale = d3.scale.quantile()
            .domain([d3.max(dataset) / 4, d3.max(dataset) / 2 , 3*d3.max(dataset) / 4, d3.max(dataset)])
            .range(["#9c9ede","#6b6ecf","#5254a3", "#393b79"]);

    svg.selectAll('div')
            .data(dataset)
            .enter()
            .append('rect')// svg doesn't have div, use rect instead
            .attr('class', "bar")
            .attr('width', xScale.rangeBand())
            .attr('x', function(each_data, index){
                return xScale(each_data);
            })
            .attr('y', function(each_data){
                return h-yScale(each_data);
            })
            .attr('height', function(each_data, i){
                return yScale(each_data);
            })
            .attr('fill', colorScale);
</script>
</body>
</html>