lib/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, 1])
        .range(["#d62728", "#fff", "#1f77b4"]);

    // 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, 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 + ")"; };
  }
})();