README.md

An example of the d3.horizon plugin.

horizon.js

(function() {
  d3.horizon = function() {
    var bands = 1, // between 1 and 5, typically
        mode = "offset", // or mirror
        interpolate = "linear", // or basis, monotone, step-before, etc.
        x = d3_horizonX,
        y = d3_horizonY,
        w = 960,
        h = 40,
        duration = 0;

    var color = d3.scale.linear()
        .domain([-1, 0, 0, 1])
        .range(["#08519c", "#bdd7e7", "#bae4b3", "#006d2c"]);

    // For each small multiple…
    function horizon(g) {
      g.each(function(d, i) {
        var g = d3.select(this),
            n = 2 * bands + 1,
            xMin = Infinity,
            xMax = -Infinity,
            yMax = -Infinity,
            x0, // old x-scale
            y0, // old y-scale
            id; // unique id for paths

        // Compute x- and y-values along with extents.
        var data = d.map(function(d, i) {
          var xv = x.call(this, d, i),
              yv = y.call(this, d, i);
          if (xv < xMin) xMin = xv;
          if (xv > xMax) xMax = xv;
          if (-yv > yMax) yMax = -yv;
          if (yv > yMax) yMax = yv;
          return [xv, yv];
        });

        // Compute the new x- and y-scales, and transform.
        var x1 = d3.scale.linear().domain([xMin, xMax]).range([0, w]),
            y1 = d3.scale.linear().domain([0, yMax]).range([0, h * bands]),
            t1 = d3_horizonTransform(bands, h, mode);

        // Retrieve the old scales, if this is an update.
        if (this.__chart__) {
          x0 = this.__chart__.x;
          y0 = this.__chart__.y;
          t0 = this.__chart__.t;
          id = this.__chart__.id;
        } else {
          x0 = x1.copy();
          y0 = y1.copy();
          t0 = t1;
          id = ++d3_horizonId;
        }

        // We'll use a defs to store the area path and the clip path.
        var defs = g.selectAll("defs")
            .data([null]);

        // The clip path is a simple rect.
        defs.enter().append("defs").append("clipPath")
            .attr("id", "d3_horizon_clip" + id)
          .append("rect")
            .attr("width", w)
            .attr("height", h);

        defs.select("rect").transition()
            .duration(duration)
            .attr("width", w)
            .attr("height", h);

        // We'll use a container to clip all horizon layers at once.
        g.selectAll("g")
            .data([null])
          .enter().append("g")
            .attr("clip-path", "url(#d3_horizon_clip" + id + ")");

        // Instantiate each copy of the path with different transforms.
        var path = g.select("g").selectAll("path")
            .data(d3.range(-1, -bands - 1, -1).concat(d3.range(1, bands + 1)), Number);

        var d0 = d3_horizonArea
            .interpolate(interpolate)
            .x(function(d) { return x0(d[0]); })
            .y0(h * bands)
            .y1(function(d) { return h * bands - y0(d[1]); })
            (data);

        var d1 = d3_horizonArea
            .x(function(d) { return x1(d[0]); })
            .y1(function(d) { return h * bands - y1(d[1]); })
            (data);

        path.enter().append("path")
            .style("fill", color)
            .attr("transform", t0)
            .attr("d", d0);

        path.transition()
            .duration(duration)
            .style("fill", color)
            .attr("transform", t1)
            .attr("d", d1);

        path.exit().transition()
            .duration(duration)
            .attr("transform", t1)
            .attr("d", d1)
            .remove();

        // Stash the new scales.
        this.__chart__ = {x: x1, y: y1, t: t1, id: id};
      });
      d3.timer.flush();
    }

    horizon.duration = function(x) {
      if (!arguments.length) return duration;
      duration = +x;
      return horizon;
    };

    horizon.bands = function(x) {
      if (!arguments.length) return bands;
      bands = +x;
      color.domain([-bands, 0, 0, bands]);
      return horizon;
    };

    horizon.mode = function(x) {
      if (!arguments.length) return mode;
      mode = x + "";
      return horizon;
    };

    horizon.colors = function(x) {
      if (!arguments.length) return color.range();
      color.range(x);
      return horizon;
    };

    horizon.interpolate = function(x) {
      if (!arguments.length) return interpolate;
      interpolate = x + "";
      return horizon;
    };

    horizon.x = function(z) {
      if (!arguments.length) return x;
      x = z;
      return horizon;
    };

    horizon.y = function(z) {
      if (!arguments.length) return y;
      y = z;
      return horizon;
    };

    horizon.width = function(x) {
      if (!arguments.length) return w;
      w = +x;
      return horizon;
    };

    horizon.height = function(x) {
      if (!arguments.length) return h;
      h = +x;
      return horizon;
    };

    return horizon;
  };

  var d3_horizonArea = d3.svg.area(),
      d3_horizonId = 0;

  function d3_horizonX(d) {
    return d[0];
  }

  function d3_horizonY(d) {
    return d[1];
  }

  function d3_horizonTransform(bands, h, mode) {
    return mode == "offset"
        ? function(d) { return "translate(0," + (d + (d < 0) - bands) * h + ")"; }
        : function(d) { return (d < 0 ? "scale(1,-1)" : "") + "translate(0," + (d - bands) * h + ")"; };
  }
})();

index.html

<style>

#horizon-controls {
  position: relative;
  width: 100%;
  font-size: 16px;
  margin-bottom: 6px;
}

#horizon-bands {
  position: absolute;
  right: 0;
}

</style>
<div id="horizon-controls">
  <input name="mode" type="radio" value="mirror" id="horizon-mode-mirror" checked><label for="horizon-mode-mirror"> Mirror</label>
  <input name="mode" type="radio" value="offset" id="horizon-mode-offset"><label for="horizon-mode-offset"> Offset</label>
  <span id="horizon-bands"><span id="horizon-bands-value">1</span> <button class="first">&#x2212;</button><button class="last">+</button></span>
</div>
<div id="horizon-chart"></div>
<script src="http://d3js.org/d3.v2.min.js?2.9.5"></script>
<script src="horizon.js?0.0.1"></script>
<script>

var width = 960,
    height = 360;

var chart = d3.horizon()
    .width(width)
    .height(height)
    .bands(1)
    .mode("mirror")
    .interpolate("basis");

var svg = d3.select("body").append("svg")
    .attr("width", width)
    .attr("height", height);

d3.json("unemployment.json", function(data) {

  // Offset so that positive is above-average and negative is below-average.
  var mean = data.rate.reduce(function(p, v) { return p + v; }, 0) / data.rate.length;

  // Transpose column values to rows.
  data = data.rate.map(function(rate, i) {
    return [Date.UTC(data.year[i], data.month[i] - 1), rate - mean];
  });

  // Render the chart.
  svg.data([data]).call(chart);

  // Enable mode buttons.
  d3.selectAll("#horizon-controls input[name=mode]").on("change", function() {
    svg.call(chart.duration(0).mode(this.value));
  });

  // Enable bands buttons.
  d3.selectAll("#horizon-bands button").data([-1, 1]).on("click", function(d) {
    var n = Math.max(1, chart.bands() + d);
    d3.select("#horizon-bands-value").text(n);
    svg.call(chart.duration(1000).bands(n).height(height / n));
  });
});

</script>

unemployment.json

{"year":[2000,2000,2000,2000,2000,2000,2000,2000,2000,2000,2000,2000,2001,2001,2001,2001,2001,2001,2001,2001,2001,2001,2001,2001,2002,2002,2002,2002,2002,2002,2002,2002,2002,2002,2002,2002,2003,2003,2003,2003,2003,2003,2003,2003,2003,2003,2003,2003,2004,2004,2004,2004,2004,2004,2004,2004,2004,2004,2004,2004,2005,2005,2005,2005,2005,2005,2005,2005,2005,2005,2005,2005,2006,2006,2006,2006,2006,2006,2006,2006,2006,2006,2006,2006,2007,2007,2007,2007,2007,2007,2007,2007,2007,2007,2007,2007,2008,2008,2008,2008,2008,2008,2008,2008,2008,2008,2008,2008,2009,2009,2009,2009,2009,2009,2009,2009,2009,2009,2009,2009,2010,2010],"month":[1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2,3,4,5,6,7,8,9,10,11,12,1,2],"rate":[4.5,4.4,4.3,3.7,3.8,4.1,4.2,4.1,3.8,3.6,3.7,3.7,4.7,4.6,4.5,4.2,4.1,4.7,4.7,4.9,4.7,5,5.3,5.4,6.3,6.1,6.1,5.7,5.5,6,5.9,5.7,5.4,5.3,5.6,5.7,6.5,6.4,6.2,5.8,5.8,6.5,6.3,6,5.8,5.6,5.6,5.4,6.3,6,6,5.4,5.3,5.8,5.7,5.4,5.1,5.1,5.2,5.1,5.7,5.8,5.4,4.9,4.9,5.2,5.2,4.9,4.8,4.6,4.8,4.6,5.1,5.1,4.8,4.5,4.4,4.8,5,4.6,4.4,4.1,4.3,4.3,5,4.9,4.5,4.3,4.3,4.7,4.9,4.6,4.5,4.4,4.5,4.8,5.4,5.2,5.2,4.8,5.2,5.7,6,6.1,6,6.1,6.5,7.1,8.5,8.9,9,8.6,9.1,9.7,9.7,9.6,9.5,9.5,9.4,9.7,10.6,10.4]}