tm_yearly_totals_meta
words_topic_yearly_plot_overall
bug fix for cite_articles
slightly less rigid tm_yearly_totals
more ways to do yearly topic-average plots

tm_yearly_line_plot: ways to adjust what is being plotted, by passing in
different numerators and denominators for the time series, as well as
fiddling the faceting.

tm_yearly_journals_plot: area plot showing how much each journal
contributes, using the results of tm_yearly_totals_meta

topic_names: utility function to get a vector of topic labels
term_year_series_frame()

Factored out from mallet_word_plot. This way you can extra a bunch of
series from different term-year-topic-matrices and plot them on a single plot.