Added a close button to the legend to close pie chart

@cschroed-usgs Merge this *after* merging NAWQAAN-312 since this pull request had to include that branch in here. 
It might be worth calling [flot's .shutdown() method](https://github.com/flot/flot/blob/master/API.md#plot-methods) here. Then again, it might not, I can't get a definitive answer on whether or not we /need/ to call that method. It may depend on whether the plugins we're using clean themselves up or not.