Fix drawing order when elements are skipped (chartjs#323)

Author: stockiNail

src/layout.js

@@ -106,7 +106,7 @@ export default {
           _hidable: false,
           _visible: true,
           _set: i,
-          _idx: j
+          _idx: label._index
         };
       }
     }

test/fixtures/drawing/draw-skipped-values-in-correct-order.js

@@ -0,0 +1,61 @@
+// https://github.com/chartjs/chartjs-plugin-datalabels/issues/319
+
+export default {
+  config: {
+    type: 'radar',
+    data: {
+      labels: ['1', '2', '3', '4', '5'],
+      datasets: [
+        {
+          label: 'Series 1',
+          data: [5, 5, 5, 5, 5],
+          backgroundColor: 'red',
+          borderWidth: 0,
+          fill: false,
+          borderColor: 'red',
+          datalabels: {
+            backgroundColor: 'red',
+          }
+        },
+        {
+          label: 'Series 2',
+          data: [4.5, 4.5, null, 4.5, 4.5],
+          backgroundColor: 'blue',
+          borderWidth: 0,
+          fill: false,
+          borderColor: 'blue',
+          datalabels: {
+            backgroundColor: 'blue',
+          }
+        }
+      ]
+    },
+    options: {
+      layout: {
+        padding: 20
+      },
+      scales: {
+        r: {
+          display: false,
+          min: 1,
+          max: 5
+        },
+      },
+      plugins: {
+        datalabels: {
+          font: {
+            size: 0
+          },
+          padding: 16,
+        }
+      }
+
+    }
+  },
+  options: {
+    canvas: {
+      height: 256,
+      width: 256
+    }
+  }
+};