pkg/js: improved readme, added example

pkg/js/example/chart_js_example.dart

Index: pkg/js/example/chart_js_example.dart
diff --git a/pkg/js/example/chart_js_example.dart b/pkg/js/example/chart_js_example.dart
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+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library chart.example;
+
+// Based off the Javascript example
+// https://github.com/nnnick/Chart.js/blob/b8691c9581bff0eeecb34f98e678dc045a18f33e/samples/line.html
+// On 2015-10-15
+
+import 'dart:html';
+import 'dart:math';
+
+import 'chart.dart';
+
+void main() {
+  var ctx = (querySelector('#canvas') as CanvasElement).context2D;
+
+  var rnd = new Random();
+
+  var data = new Data(labels: [
+    "January",
+    "February",
+    "March",
+    "April",
+    "May",
+    "June",
+    "July"
+  ], datasets: <DataSet>[
+    new DataSet(
+        label: "My First dataset",
+        fillColor: "rgba(220,220,220,0.2)",
+        strokeColor: "rgba(220,220,220,1)",
+        pointColor: "rgba(220,220,220,1)",
+        pointStrokeColor: "#fff",
+        pointHighlightFill: "#fff",
+        pointHighlightStroke: "rgba(220,220,220,1)",
+        data: [
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100)
+        ]),
+    new DataSet(
+        label: "My Second dataset",
+        fillColor: "rgba(151,187,205,0.2)",
+        strokeColor: "rgba(151,187,205,1)",
+        pointColor: "rgba(151,187,205,1)",
+        pointStrokeColor: "#fff",
+        pointHighlightFill: "#fff",
+        pointHighlightStroke: "rgba(151,187,205,1)",
+        data: [
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100),
+          rnd.nextInt(100)
+        ])
+  ]);
+
+  new Chart(ctx).Line(data, new Options(responsive: true));
+}