Add sample how to select annotations (chartjs#751)

* Add sample how to select annotations

* Clean up the useless references

* changes the sample a little bit

docs/.vuepress/config.js

@@ -184,6 +184,7 @@ module.exports = {
           children: [
             'interaction/interaction',
             'interaction/dragging',
+            'interaction/selection',
           ],
         },
         'utils',

docs/samples/interaction/dragging.md

@@ -130,7 +130,7 @@ const handleDrag = function(event) {
     }
   }
 };
-// </block:dragger>
+// </block:utils>
 
 // <block:dragger:5>
 const dragger = {

docs/samples/interaction/selection.md

@@ -0,0 +1,140 @@
+# Selecting annotations
+
+```js chart-editor
+// <block:setup:3>
+let count = 0;
+const selected = [];
+
+Utils.srand(8);
+
+const data = {
+  labels: ['January', 'February', 'March', 'April', 'May', 'June', 'July'],
+  datasets: [{
+    type: 'line',
+    label: 'Dataset 1',
+    borderColor: 'rgb(54, 162, 235)',
+    borderWidth: 2,
+    fill: false,
+    data: Utils.numbers({count: 7, min: 0, max: 100}),
+  }]
+};
+// </block:setup>
+
+// <block:annotation1:1>
+const annotation1 = {
+  type: 'box',
+  backgroundColor: 'rgba(255, 245, 157, 0.2)',
+  borderColor: 'rgb(255, 245, 157)',
+  borderWidth: 2,
+  click: ({element}) => select(element, 'rgba(255, 245, 157, 0.8)', 'rgba(255, 245, 157, 0.2)'),
+  label: {
+    display: true,
+    content: 'Yellow box annotation',
+    position: {
+      y: 'start'
+    },
+    font: {
+      size: 12
+    }
+  },
+  xMax: 'April',
+  xMin: 'February',
+  xScaleID: 'x',
+  yMax: 90,
+  yMin: 10,
+  yScaleID: 'y'
+};
+// </block:annotation1>
+
+// <block:annotation2:2>
+const annotation2 = {
+  type: 'box',
+  backgroundColor: 'rgba(165, 214, 167, 0.2)',
+  borderColor: 'rgb(165, 214, 167)',
+  borderWidth: 2,
+  click: ({element}) => select(element, 'rgba(165, 214, 167, 0.8)', 'rgba(165, 214, 167, 0.2)'),
+  label: {
+    display: true,
+    content: 'Green box annotation',
+    position: {
+      y: 'start'
+    },
+    font: {
+      size: 12
+    }
+  },
+  xMax: 'May',
+  xMin: 'March',
+  xScaleID: 'x',
+  yMax: 75,
+  yMin: 25,
+  yScaleID: 'y'
+};
+// </block:annotation2>
+
+// <block:utils:4>
+function enter({chart, element}) {
+  console.log(element.label.options.content + ' entered');
+  if (!count) {
+    chart.canvas.style.cursor = 'pointer';
+  }
+  count++;
+}
+
+function leave({chart, element}) {
+  console.log(element.label.options.content + ' left');
+  count--;
+  if (!count) {
+    chart.canvas.style.cursor = 'default';
+  }
+}
+
+function select(element, selectedColor, unselectedColor) {
+  console.log(element.label.options.content + ' selected');
+  if (selected.includes(element)) {
+    selected.splice(selected.indexOf(element), 1);
+    element.options.backgroundColor = unselectedColor;
+    element.label.options.font.size = 12;
+  } else {
+    selected.push(element);
+    element.options.backgroundColor = selectedColor;
+    element.label.options.font.size = 14;
+  }
+  return true;
+}
+// </block:utils>
+
+/* <block:config:0> */
+const config = {
+  type: 'line',
+  data,
+  options: {
+    scales: {
+      y: {
+        beginAtZero: true,
+        min: 0,
+        max: 100
+      }
+    },
+    plugins: {
+      annotation: {
+        enter: enter,
+        leave: leave,
+        common: {
+          drawTime: 'beforeDraw'
+        },
+        annotations: {
+          annotation1,
+          annotation2
+        }
+      }
+    }
+  }
+};
+/* </block:config> */
+
+module.exports = {
+  config: config,
+  output: 'console.log output is shown here, click one of the annotations'
+};
+```