Series chart tests/docs and preprocessData bug fix

SeriesChart tested and documented :)

_preprocessData needs to happen before _generateG in coordinateChart, otherwise series chart options are not applied to subcharts (for instance, transitionDuration, which needs to be 0 for tests).

Also, we left valueSort undocumented and untested. We don't understand the functionality of this option. Changing the keySort function to descending does not seem to change the graph in any way. Can this be taken out?