Update getShapeOffset() to allow data series with unequal amount of x-values

src/shape.js

@@ -41,10 +41,25 @@ c3_chart_internal_fn.getShapeOffset = function (typeFilter, indices, isSub) {
             var values = $$.isStepType(d) ? $$.convertValuesToStep(t.values) : t.values;
             if (t.id === d.id || indices[t.id] !== indices[d.id]) { return; }
             if (targetIds.indexOf(t.id) < targetIds.indexOf(d.id)) {
+                // check if the x values line up
+                if (typeof values[i] === 'undefined' ||
+                    (values[i].x !== d.x) && (values[i].x - d.x !== 0)) {
+                    // if not, try to find the value that does line up
+                    i = -1;
+                    for (var j in values) {
+                        if ((values[j].x === d.x) || (values[j].x - d.x === 0)) {
+                            i = j;
+                            break;
+                        }
+                    }
+                }
+
+                if (i in values) {
                     if (values[i].value * d.value >= 0) {
                         offset += scale(values[i].value) - y0;
                     }
                 }
+            }
         });
         return offset;
     };