soft-matter/trackpy — pull request #166 by danielballan (2014-10-05 19:56:28+00:00)
Title: ENH: Helpful error on plot_traj(empty_df)

A minor improvement:

`plot_traj(accidentally_empty_df)` used to raise the cryptic error:

AttributeError: 'DataFrame' object has no attribute 'find'

which has bitten my undergrads a couple times. Now, a more helpful `ValueError` is raised instead. Test included.