Ameliorate lag in the legend UI

[WillHannon-MCB]

There is a significant amount of lag when selecting conditions on the chart. Some of this has to do with the logic of the application. This pull request does the following:

1. The legend is automatically updated by requesting animation frames before updating the application state.
2. Selecting single conditions is expedited by adding an event listener for double-click events.

The consequence of these changes is that the legend appears less laggy and the user can select a single condition while deselecting all others by double-clicking on that condition in the legend.