Accept data frames as input for plot_coefs

Will require checking whether all the necessary components are in the data frame and then not running `tidy` on them without totally mucking up the code.

Start at the plot_coefs entry point and trace how its current input is passed through tidy. Identify which components a data frame must contain, then verify that data-frame inputs avoid the inappropriate tidy path while preserving existing behavior. Done means plot_coefs accepts valid data frames without breaking its current inputs.

Written by the indexing model from the issue text.