Store plot history within a table

[mDibyo]

In addition to making it easier to manipulate the plots, this will also make it easier to test the plotting functions. See discussion in #132.

[SamLau95]

I actually am liking this idea more now because without this we can't test whether students have called hist properly during lab and projects.

[papajohn]

It's too bad that the notebook will then display the object using <...> syntax before the visualization. We could instead store the history of plots as a field in the table. Thoughts?