ENH: Improve plot_traj

Expose `vmin` and `vmax`, and support superimposition without calibration,
which previously raised a `NotImplementedError`.
But, to Tom's original point, what happens if TeX isn't installed? On my system, `u'µm'` shows up as an unprintable character (the empty box).

I'm OK with going with `$\mu$` though. Users who don't have TeX and write their own axes labels. Is that the best compromise?