Take a set of values and get the raw materials for making a Lorenz Curve.

README.md

Example: http://jsbin.com/hamur

lorenz.js

var lorenz = function (data) {
  
  // Sort the data in ascending order.
  data = data.sort(function (a,b) {
	return d3.ascending(a, b);
  });
  
  // Divide into quintiles.
  // - Create an array of five empty sub-arrays.
  var quintiles = Array.apply(null, new Array(5)).map(function () { return []; });
  // - Create a scale and put each value in the right sub-array by its index.
  var scale = d3.scale.quantize()
                      .domain([0, data.length])
                      .range(d3.range(0,5));
  data.forEach(function (datum, index) {
    quintiles[scale(index)].push(datum);
  });
  
  // Get the total for each quintile.
  return quintiles.map(function (quintile) {
    return d3.sum(quintile);
  })
  // Calculate the percent of total income in each quintile.
  .map(function (quintile) {
    return quintile / d3.sum(data);
  })
  // Calculate the cumulative percentage of household income.
  .map(function (quintile, index, data) {
    return quintile + d3.sum(data.slice(0, index));
   });
  
};

reusable-chart.js

var d = [1,2,3,4,5,6,7,8,9,10,11,12,13,18,20];

var lorenz = function (data) {
  
  // Sort the data in ascending order.
  data = data.sort(function (a,b) {
	return d3.ascending(a, b);
  });
  
  // Divide into quintiles.
  // - Create an array of five empty sub-arrays.
  var quintiles = Array.apply(null, new Array(5)).map(function () { return []; });
  // - Create a scale and put each value in the right sub-array by its index.
  var scale = d3.scale.quantize().domain([0, data.length]).range(d3.range(0,5));
  data.forEach(function (datum, index) {
    quintiles[scale(index)].push(datum);
  });
  
  // Get the total for each quintile.
  return quintiles.map(function (quintile) {
    return d3.sum(quintile);
  })
  // Calculate the percent of total income in each quintile.
  .map(function (quintile) {
    return quintile / d3.sum(data);
  })
  // Calculate the cumulative percentage of household income.
  .map(function (quintile, index, data) {
    return quintile + d3.sum(data.slice(0, index));
   });
  
};

var lorenzify = function () {
  return Array.prototype.slice.call(arguments).map(function (data) {
    return lorenz(data);
  });
};

var dataset = lorenz(d);

function lorenzCurve(options) {
  
  var dataset = options.data;
  
  var colors = options.colors || [
    '#F00', '#0F0', '#00F', '#F0F'
  ];
  var ci = 0;
  
  var width = options.height || 500;
  var height = options.weight || 250;
  var margin = options.margin || 45;

  var svg = d3.select(options.el || '#chart')
    .append('svg')
    .attr({
      width: width,
      height: height
    });

  var xScale = d3.scale.ordinal()
    .domain(d3.range(5))
    .rangeBands([0, width - (margin * 2)], 1, 0);

  var yScale = d3.scale.linear()
    .range([height - (margin * 2), 0]);

  var line = d3.svg.line()
    .x(function(d,i) { return xScale(i); })
    .y(function(d,i) { return yScale(d); });

  dataset.forEach(function (d) {
    svg.append('path')
    .datum(d)
    .attr('fill', 'none')
    .attr('stroke', colors[ci++])
    .attr('d', line)
    .attr('transform', 'translate(' + margin + ',' + margin + ')');
  });

  var yAxis = d3.svg.axis()
    .scale(yScale)
    .orient('left')
    .ticks(5)
    .tickFormat(function(d) { return d * 100 + '%'; });

  svg.append('g')
    .attr('class', 'axis')
    .attr('transform', 'translate(' + (margin * 0.9) + ',' + margin + ')')
    .call(yAxis);

  var xAxis = d3.svg.axis()
    .scale(xScale)
    .orient('bottom')
    .ticks(5);

  svg.append('g')
    .attr('class', 'axis')
    .attr('transform', 'translate(' + margin + ',' + (height - margin * 0.9) + ')')
    .call(xAxis);
  
  return svg;
}

lorenzCurve({ data: [dataset] });