a ggplot2 object that you can further customize with scales, labels, etc.
Arguments
vote_tally
either a pprc object (the result of a call to roll_call()) or
a data.frame of vote tallies for the senate. It expects 3 columns. state_abbrev : the
2-letter U.S. state abbreviation; district : either 1 or 2 to distinguish between
each senator; party : R, D or ID; position : yes, no, present, none for
how the senator voted.