adding step-middle as interpolation method

src/svg/line.js

@@ -92,6 +92,7 @@ var d3_svg_lineInterpolators = d3.map({
   "linear": d3_svg_lineLinear,
   "linear-closed": d3_svg_lineLinearClosed,
   "step-before": d3_svg_lineStepBefore,
+  "step-middle": d3_svg_lineStepMiddle,
   "step-after": d3_svg_lineStepAfter,
   "basis": d3_svg_lineBasis,
   "basis-open": d3_svg_lineBasisOpen,
@@ -127,6 +128,16 @@ function d3_svg_lineStepBefore(points) {
   return path.join("");
 }
 
+// Step interpolation; generates "H" and "V" commands.
+function d3_svg_lineStepMiddle(points) {
+  var i = 0, n = points.length, p = points[0], path = [ p[0], ",", p[1] ];
+  while (++i < n) {
+    var prev = points[i-1], p = points[i];
+    path.push("H", (p[0] - prev[0])/2 + prev[0], "V", p[1], "H", p[0]);
+  }
+  return path.join("");
+}
+
 // Step interpolation; generates "H" and "V" commands.
 function d3_svg_lineStepAfter(points) {
   var i = 0,

test/svg/line-test.js

@@ -93,6 +93,12 @@ suite.addBatch({
         assert.pathEqual(l([[0, 0], [1, 1]]), "M0,0V1H1");
         assert.pathEqual(l([[0, 0], [1, 1], [2, 0]]), "M0,0V1H1V0H2");
       },
+      "supports step-middle interpolation": function(line) {
+        var l = line().interpolate("step-middle");
+        assert.pathEqual(l([[0, 0]]), "M0,0");
+        assert.pathEqual(l([[0, 0], [1, 1]]), "M0,0H0.5V1H1");
+        assert.pathEqual(l([[0, 0], [1, 1], [2, 0]]), "M0,0H0.5V1H1H1.5V0H2");
+      },
       "supports step-after interpolation": function(line) {
         var l = line().interpolate("step-after");
         assert.pathEqual(l([[0, 0]]), "M0,0");