Fix negatively-sized rectangles in treemap.

Includes unit test!

Fixes #136.

d3.layout.js

@@ -1754,7 +1754,7 @@ d3.layout.treemap = function() {
         v = u ? round(row.area / u) : 0,
         o;
     if (u == rect.dx) { // horizontal subdivision
-      if (flush || v > rect.dy) v = rect.dy; // over+underflow
+      if (flush || v > rect.dy) v = v ? rect.dy : 0; // over+underflow
       while (++i < n) {
         o = row[i];
         o.x = x;
@@ -1767,7 +1767,7 @@ d3.layout.treemap = function() {
       rect.y += v;
       rect.dy -= v;
     } else { // vertical subdivision
-      if (flush || v > rect.dx) v = rect.dx; // over+underflow
+      if (flush || v > rect.dx) v = v ? rect.dx : 0; // over+underflow
       while (++i < n) {
         o = row[i];
         o.x = x;

src/layout/treemap.js

@@ -106,7 +106,7 @@ d3.layout.treemap = function() {
         v = u ? round(row.area / u) : 0,
         o;
     if (u == rect.dx) { // horizontal subdivision
-      if (flush || v > rect.dy) v = rect.dy; // over+underflow
+      if (flush || v > rect.dy) v = v ? rect.dy : 0; // over+underflow
       while (++i < n) {
         o = row[i];
         o.x = x;
@@ -119,7 +119,7 @@ d3.layout.treemap = function() {
       rect.y += v;
       rect.dy -= v;
     } else { // vertical subdivision
-      if (flush || v > rect.dx) v = rect.dx; // over+underflow
+      if (flush || v > rect.dx) v = v ? rect.dx : 0; // over+underflow
       while (++i < n) {
         o = row[i];
         o.x = x;

test/layout/treemap-test.js

@@ -134,6 +134,36 @@ suite.addBatch({
         {x: 1, y: 556, dx: 599, dy: 277},
         {x: 1, y: 1, dx: 599, dy: 555}
       ]);
+    },
+    "no negatively sized rectangles": function(treemap) {
+      var t = treemap().size([615, 500]).sort(function(a, b) { return a.value - b.value; }).padding(29),
+          nodes = t.nodes({"children": [
+            {"value": 1},
+            {"value": 9},
+            {"value": 3},
+            {"value": 15},
+            {"value": 44},
+            {"value": 28},
+            {"value": 32},
+            {"value": 41},
+            {"value": 50},
+            {"value": 60},
+            {"value": 64},
+            {"value": 75},
+            {"value": 76},
+            {"value": 84},
+            {"value": 88},
+            {"value": 100},
+            {"value": 140},
+            {"value": 142},
+            {"value": 363},
+            {"value": 657},
+            {"value": 670},
+            {"value": 822},
+            {"value": 1173},
+            {"value": 1189}
+          ]}).map(layout);
+      assert.equal(nodes.filter(function(n) { return n.dx < 0 || n.dy < 0; }).length, 0);
     }
   }
 });