d3/_site/src/scale/linear.js

A small, free JavaScript library for manipulating documents based on data.

d3.scale.linear = function() {
  var x0 = 0,
      x1 = 1,
      y0 = 0,
      y1 = 1,
      kx = 1, // 1 / (x1 - x0)
      ky = 1, // (x1 - x0) / (y1 - y0)
      interpolate = d3.interpolate,
      i = interpolate(y0, y1),
      clamp = false;

  function scale(x) {
    x = (x - x0) * kx;
    return i(clamp ? Math.max(0, Math.min(1, x)) : x);
  }

  // Note: requires range is coercible to number!
  scale.invert = function(y) {
    return (y - y0) * ky + x0;
  };

  scale.domain = function(x) {
    if (!arguments.length) return [x0, x1];
    x0 = +x[0];
    x1 = +x[1];
    kx = 1 / (x1 - x0);
    ky = (x1 - x0) / (y1 - y0);
    return scale;
  };

  scale.range = function(x) {
    if (!arguments.length) return [y0, y1];
    y0 = x[0];
    y1 = x[1];
    ky = (x1 - x0) / (y1 - y0);
    i = interpolate(y0, y1);
    return scale;
  };

  scale.rangeRound = function(x) {
    return scale.range(x).interpolate(d3.interpolateRound);
  };

  scale.clamp = function(x) {
    if (!arguments.length) return clamp;
    clamp = x;
    return scale;
  };

  scale.interpolate = function(x) {
    if (!arguments.length) return interpolate;
    i = (interpolate = x)(y0, y1);
    return scale;
  };

  // TODO Dates? Ugh.
  function tickRange(m) {
    var start = Math.min(x0, x1),
        stop = Math.max(x0, x1),
        span = stop - start,
        step = Math.pow(10, Math.floor(Math.log(span / m) / Math.LN10)),
        err = m / (span / step);

    // Filter ticks to get closer to the desired count.
    if (err <= .15) step *= 10;
    else if (err <= .35) step *= 5;
    else if (err <= .75) step *= 2;

    // Round start and stop values to step interval.
    return {
      start: Math.ceil(start / step) * step,
      stop: Math.floor(stop / step) * step + step * .5, // inclusive
      step: step
    };
  }

  scale.ticks = function(m) {
    var range = tickRange(m);
    return d3.range(range.start, range.stop, range.step);
  };

  scale.tickFormat = function(m) {
    var n = Math.max(0, -Math.floor(Math.log(tickRange(m).step) / Math.LN10 + .01));
    return d3.format(",." + n + "f");
  };

  return scale;
};