nuke cgp_hist_plot in favor of alt_cgp_hist_plot

[almartin82]

This is a breaking change, so I wanted to open it up for discussion before I did it.

Right now cgp_hist_plot shows a cohort trajectory against the average student percentile rank. With the advent of the 2015 norms, NWEA now reports a grade/cohort status percentile rank. That is to say, we can track the percentile rank of a class, compared to other classes.

Here's the old way (average student percentile):

Here's the new way (currently alt_cgp_hist_plot)

I say that we make alt_cgp_hist_plot the only type of cgp_hist_plot. @chrishaid what say you?

[almartin82]

ps this issue filed on a :airplane:!

[almartin82]

nb, pull request #256 currently preserves both versions of the plot. @chrishaid if you agree with this proposal I'll turn this into a task.

[almartin82]

@chrishaid can I bump this one and get your thoughts here?