The lines

        xs, ys, zs = np.broadcast_arrays(
            *[np.ravel(np.ma.filled(t, np.nan)) for t in [xs, ys, zs]])

at the top of the function ensure that zs is already broadcasted against
xs, and always 1D (and thus iterable); hence, the later iterability check
and broadcasting are unnecessary.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way