"Bundle" interpolation for single-element arrays.

In future we may want to generate some kind of loop, but it's not clear
what orientation such a loop should have, so perhaps a "non-line" like
this is better as a default.

d3.v2.js

@@ -3547,19 +3547,21 @@ function d3_svg_lineBasisClosed(points) {
 }
 
 function d3_svg_lineBundle(points, tension) {
-  var n = points.length - 1,
-      x0 = points[0][0],
-      y0 = points[0][1],
-      dx = points[n][0] - x0,
-      dy = points[n][1] - y0,
-      i = -1,
-      p,
-      t;
-  while (++i <= n) {
-    p = points[i];
-    t = i / n;
-    p[0] = tension * p[0] + (1 - tension) * (x0 + t * dx);
-    p[1] = tension * p[1] + (1 - tension) * (y0 + t * dy);
+  var n = points.length - 1;
+  if (n) {
+    var x0 = points[0][0],
+        y0 = points[0][1],
+        dx = points[n][0] - x0,
+        dy = points[n][1] - y0,
+        i = -1,
+        p,
+        t;
+    while (++i <= n) {
+      p = points[i];
+      t = i / n;
+      p[0] = tension * p[0] + (1 - tension) * (x0 + t * dx);
+      p[1] = tension * p[1] + (1 - tension) * (y0 + t * dy);
+    }
   }
   return d3_svg_lineBasis(points);
 }

src/svg/line.js

@@ -298,19 +298,21 @@ function d3_svg_lineBasisClosed(points) {
 }
 
 function d3_svg_lineBundle(points, tension) {
-  var n = points.length - 1,
-      x0 = points[0][0],
-      y0 = points[0][1],
-      dx = points[n][0] - x0,
-      dy = points[n][1] - y0,
-      i = -1,
-      p,
-      t;
-  while (++i <= n) {
-    p = points[i];
-    t = i / n;
-    p[0] = tension * p[0] + (1 - tension) * (x0 + t * dx);
-    p[1] = tension * p[1] + (1 - tension) * (y0 + t * dy);
+  var n = points.length - 1;
+  if (n) {
+    var x0 = points[0][0],
+        y0 = points[0][1],
+        dx = points[n][0] - x0,
+        dy = points[n][1] - y0,
+        i = -1,
+        p,
+        t;
+    while (++i <= n) {
+      p = points[i];
+      t = i / n;
+      p[0] = tension * p[0] + (1 - tension) * (x0 + t * dx);
+      p[1] = tension * p[1] + (1 - tension) * (y0 + t * dy);
+    }
   }
   return d3_svg_lineBasis(points);
 }

test/svg/line-test.js

@@ -134,6 +134,10 @@ suite.addBatch({
       "observes the specified tension": function(line) {
         var l = line().interpolate("bundle").tension(1);
         assert.pathEqual(l([[0, 0], [1, 1], [2, 0], [3, 1], [4, 0]]), line().interpolate("basis")([[0, 0], [1, 1], [2, 0], [3, 1], [4, 0]]));
+      },
+      "supports a single-element array": function(line) {
+        var l = line().interpolate("bundle").tension(1);
+        assert.pathEqual(l([[0, 0]]), "M0,0");
       }
     },