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dygraphs plugins
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A single dygraph is actually a collection of dygraphs plugins, each responsible
for some portion of the chart: the plot area, the axes, the legend, the labels,
etc.

This forces the dygraphs code to be more modular and encourages better APIs.

The "legend" plugin (plugins/legend.js) can be used as a template for new
plugins.

Here is a simplified version of it, with comments to explain the plugin
lifecycle and APIs:

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// (standard JavaScript library wrapper; prevents polluting global namespace)
Dygraph.Plugins.Legend = (function() {

// Plugin constructor. This is invoked once for every chart which uses the
// plugin. You can't actually access the Dygraph object at this point, so the
// initialization you do here shouldn't be chart-specific. (For that, use
// the activate() method).
var legend = function() {
  this.div_ = null;
};

// Plugins are expected to implement this method for debugging purposes.
legend.toString = function() {
  return "Legend";
};

// This is called once the dygraph is ready. The chart data may not be
// available yet, but the options specified in the constructor are.
// 
// Proper tasks to do here include:
// - Reading your own options
// - DOM manipulation
// - Registering event listeners
//
// "dygraph" is the Dygraph object for which this instance is being activated.
// "registerer" allows you to register event listeners.
legend.prototype.activate = function(dygraph, registerer) {
  // Create the legend div and attach it to the chart DOM.
  this.div_ = document.createElement("div");
  dygraph.graphDiv.appendChild(this.div_);

  // Add event listeners. These will be called whenever points are selected
  // (i.e. you hover over them) or deselected (i.e. you mouse away from the
  // chart). This is your only chance to register event listeners! Once this
  // method returns, the gig is up.
  registerer.addEventListener('select', legend.select);
  registerer.addEventListener('deselect', legend.deselect);
};

// The functions called by event listeners all take a single parameter, an
// event object. This contains properties relevant to the particular event, but
// you can always assume that it has:
//
// 1. A "dygraph" parameter which is a reference to the chart on which the
//    event took place.
// 2. A "stopPropagation" method, which you can call to prevent the event from
//    being seen by any other plugins after you. This effectively cancels the
//    event.
// 3. A "preventDefault" method, which prevents dygraphs from performing the
//    default action associated with this event.
//
legend.select = function(e) {
  // These are two of the properties specific to the "select" event object:
  var xValue = e.selectedX;
  var points = e.selectedPoints;

  var html = xValue + ':';
  for (var i = 0; i < points.length; i++) {
    var point = points[i];
    html += ' ' + point.name + ':' + point.yval;
  }

  // In an event listener, "this" refers to your plugin object.
  this.div_.innerHTML = html;
};

// This clears out the legend when the user mouses away from the chart.
legend.deselect = function(e) {
  this.div_.innerHTML = '';
};

return legend;
})();

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Plugin Events Reference:

- predraw
- clearChart
- drawChart
- select
- deselect

TODO(danvk): document all event properties for each event.


Special methods:
- (constructor)
- activate
- destroy


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Notes on plugin registration and event cascade ordering/behavior.