source: Dev/trunk/d3/src/layout/stack.js @ 76

// data is two-dimensional array of x,y; we populate y0
d3.layout.stack = function() {
  var values = Object,
      order = d3_layout_stackOrders["default"],
      offset = d3_layout_stackOffsets["zero"],
      out = d3_layout_stackOut,
      x = d3_layout_stackX,
      y = d3_layout_stackY;

  function stack(data, index) {

    // Convert series to canonical two-dimensional representation.
    var series = data.map(function(d, i) {
      return values.call(stack, d, i);
    });

    // Convert each series to canonical [[x,y]] representation.
    var points = series.map(function(d, i) {
      return d.map(function(v, i) {
        return [x.call(stack, v, i), y.call(stack, v, i)];
      });
    });

    // Compute the order of series, and permute them.
    var orders = order.call(stack, points, index);
    series = d3.permute(series, orders);
    points = d3.permute(points, orders);

    // Compute the baseline

    var offsets = offset.call(stack, points, index);

    // And propagate it to other series.
    var n = series.length,
        m = series[0].length,
        i,
        j,
        o;
    for (j = 0; j < m; ++j) {
      out.call(stack, series[0][j], o = offsets[j], points[0][j][1]);
      for (i = 1; i < n; ++i) {
        out.call(stack, series[i][j], o += points[i - 1][j][1], points[i][j][1]);
      }
    }

    return data;
  }

  stack.values = function(x) {
    if (!arguments.length) return values;
    values = x;
    return stack;
  };

  stack.order = function(x) {
    if (!arguments.length) return order;
    order = typeof x === "function" ? x : d3_layout_stackOrders[x];
    return stack;
  };

  stack.offset = function(x) {
    if (!arguments.length) return offset;
    offset = typeof x === "function" ? x : d3_layout_stackOffsets[x];
    return stack;
  };

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

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

  stack.out = function(z) {
    if (!arguments.length) return out;
    out = z;
    return stack;
  };

  return stack;
}

function d3_layout_stackX(d) {
  return d.x;
}

function d3_layout_stackY(d) {
  return d.y;
}

function d3_layout_stackOut(d, y0, y) {
  d.y0 = y0;
  d.y = y;
}

var d3_layout_stackOrders = {

  "inside-out": function(data) {
    var n = data.length,
        i,
        j,
        max = data.map(d3_layout_stackMaxIndex),
        sums = data.map(d3_layout_stackReduceSum),
        index = d3.range(n).sort(function(a, b) { return max[a] - max[b]; }),
        top = 0,
        bottom = 0,
        tops = [],
        bottoms = [];
    for (i = 0; i < n; ++i) {
      j = index[i];
      if (top < bottom) {
        top += sums[j];
        tops.push(j);
      } else {
        bottom += sums[j];
        bottoms.push(j);
      }
    }
    return bottoms.reverse().concat(tops);
  },

  "reverse": function(data) {
    return d3.range(data.length).reverse();
  },

  "default": function(data) {
    return d3.range(data.length);
  }

};

var d3_layout_stackOffsets = {

  "silhouette": function(data) {
    var n = data.length,
        m = data[0].length,
        sums = [],
        max = 0,
        i,
        j,
        o,
        y0 = [];
    for (j = 0; j < m; ++j) {
      for (i = 0, o = 0; i < n; i++) o += data[i][j][1];
      if (o > max) max = o;
      sums.push(o);
    }
    for (j = 0; j < m; ++j) {
      y0[j] = (max - sums[j]) / 2;
    }
    return y0;
  },

  "wiggle": function(data) {
    var n = data.length,
        x = data[0],
        m = x.length,
        max = 0,
        i,
        j,
        k,
        s1,
        s2,
        s3,
        dx,
        o,
        o0,
        y0 = [];
    y0[0] = o = o0 = 0;
    for (j = 1; j < m; ++j) {
      for (i = 0, s1 = 0; i < n; ++i) s1 += data[i][j][1];
      for (i = 0, s2 = 0, dx = x[j][0] - x[j - 1][0]; i < n; ++i) {
        for (k = 0, s3 = (data[i][j][1] - data[i][j - 1][1]) / (2 * dx); k < i; ++k) {
          s3 += (data[k][j][1] - data[k][j - 1][1]) / dx;
        }
        s2 += s3 * data[i][j][1];
      }
      y0[j] = o -= s1 ? s2 / s1 * dx : 0;
      if (o < o0) o0 = o;
    }
    for (j = 0; j < m; ++j) y0[j] -= o0;
    return y0;
  },

  "expand": function(data) {
    var n = data.length,
        m = data[0].length,
        k = 1 / n,
        i,
        j,
        o,
        y0 = [];
    for (j = 0; j < m; ++j) {
      for (i = 0, o = 0; i < n; i++) o += data[i][j][1];
      if (o) for (i = 0; i < n; i++) data[i][j][1] /= o;
      else for (i = 0; i < n; i++) data[i][j][1] = k;
    }
    for (j = 0; j < m; ++j) y0[j] = 0;
    return y0;
  },

  "zero": function(data) {
    var j = -1,
        m = data[0].length,
        y0 = [];
    while (++j < m) y0[j] = 0;
    return y0;
  }

};

function d3_layout_stackMaxIndex(array) {
  var i = 1,
      j = 0,
      v = array[0][1],
      k,
      n = array.length;
  for (; i < n; ++i) {
    if ((k = array[i][1]) > v) {
      j = i;
      v = k;
    }
  }
  return j;
}

function d3_layout_stackReduceSum(d) {
  return d.reduce(d3_layout_stackSum, 0);
}

function d3_layout_stackSum(p, d) {
  return p + d[1];
}