Issue #263. Fixed bug in stackedArea where the data[i].values array was being set to undefined.

src/models/stackedArea.js

@@ -61,9 +61,10 @@ nv.models.stackedArea = function() {
       // Injecting point index into each point because d3.layout.stack().out does not give index
       data.forEach(function(aseries, i) {
         aseries.seriesIndex = i;
-        aseries.values = aseries.values.forEach(function(d, j) {
+        aseries.values = aseries.values.map(function(d, j) {
           d.index = j;
           d.seriesIndex = i;
+          return d;
         });
       });