Updates to sorting and mean calculations

Refactoring to sorting and averaging to fix some bugs in the new front page charts.

I also added a showPoints() function to LineSeries to make the points optional. Longer term, this could take an options argument for how the points should be displayed.
The description of the compareFunction needs rewording; it doesn't make sense on its own. I think we need to mention how it behaves when it returns ==0 and <0 for some cases, as well as >0.

Also, should this.computeFunction be this.compareFunction?