Render line segments with a single point.

Fixes #2061. A line segment with a single point is now rendered as "M2,3Z"
rather than "M2,3", such that if there is an associated stroke-linecap, it is
displayed correctly.

d3.js

@@ -7344,10 +7344,8 @@
     }
     function treemap(d) {
       var nodes = stickies || hierarchy(d), root = nodes[0];
-      root.x = 0;
-      root.y = 0;
-      root.dx = size[0];
-      root.dy = size[1];
+      root.x = root.y = 0;
+      if (root.value) root.dx = size[0], root.dy = size[1]; else root.dx = root.dy = 0;
       if (stickies) hierarchy.revalue(root);
       scale([ root ], root.dx * root.dy / root.value);
       (stickies ? stickify : squarify)(root);
@@ -8184,10 +8182,10 @@
     value.closed = /-closed$/.test(key);
   });
   function d3_svg_lineLinear(points) {
-    return points.join("L");
+    return points.length > 1 ? points.join("L") : points + "Z";
   }
   function d3_svg_lineLinearClosed(points) {
-    return d3_svg_lineLinear(points) + "Z";
+    return points.join("L") + "Z";
   }
   function d3_svg_lineStep(points) {
     var i = 0, n = points.length, p = points[0], path = [ p[0], ",", p[1] ];
@@ -8209,7 +8207,7 @@
     return points.length < 4 ? d3_svg_lineLinear(points) : points[1] + d3_svg_lineHermite(points.slice(1, -1), d3_svg_lineCardinalTangents(points, tension));
   }
   function d3_svg_lineCardinalClosed(points, tension) {
-    return points.length < 3 ? d3_svg_lineLinear(points) : points[0] + d3_svg_lineHermite((points.push(points[0]), 
+    return points.length < 3 ? d3_svg_lineLinearClosed(points) : points[0] + d3_svg_lineHermite((points.push(points[0]), 
     points), d3_svg_lineCardinalTangents([ points[points.length - 2] ].concat(points, [ points[1] ]), tension));
   }
   function d3_svg_lineCardinal(points, tension) {

src/svg/line.js

@@ -104,11 +104,11 @@ d3_svg_lineInterpolators.forEach(function(key, value) {
 
 // Linear interpolation; generates "L" commands.
 function d3_svg_lineLinear(points) {
-  return points.join("L");
+  return points.length > 1 ? points.join("L") : points + "Z";
 }
 
 function d3_svg_lineLinearClosed(points) {
-  return d3_svg_lineLinear(points) + "Z";
+  return points.join("L") + "Z";
 }
 
 // Step interpolation; generates "H" and "V" commands.
@@ -153,7 +153,7 @@ function d3_svg_lineCardinalOpen(points, tension) {
 // Closed cardinal spline interpolation; generates "C" commands.
 function d3_svg_lineCardinalClosed(points, tension) {
   return points.length < 3
-      ? d3_svg_lineLinear(points)
+      ? d3_svg_lineLinearClosed(points)
       : points[0] + d3_svg_lineHermite((points.push(points[0]), points),
         d3_svg_lineCardinalTangents([points[points.length - 2]]
         .concat(points, [points[1]]), tension));

test/svg/line-test.js

@@ -149,7 +149,7 @@ suite.addBatch({
       },
       "supports a single-element array": function(line) {
         var l = line().interpolate("bundle").tension(1);
-        assert.pathEqual(l([[0, 0]]), "M0,0");
+        assert.pathEqual(l([[0, 0]]), "M0,0Z");
       }
     },