ENH: Improve plot_traj

Expose `vmin` and `vmax`, and support superimposition without calibration,
which previously raised a `NotImplementedError`.
I'm actually leaning toward $\mu$ :). As you say, the point is to get something intelligible up on the screen, not to be publication-quality. My guess is that the Unicode char only works with certain fonts, which might vary by platform or backend.

On Aug 18, 2014, at 1:52 PM, Dan Allan <notifications@github.com> wrote:

> In trackpy/plots.py:
> 
> >      else:
> > -        ax.set_xlabel(r'x [$\mu$m]')
> > -        ax.set_ylabel(r'y [$\mu$m]')
> > +        ax.set_xlabel(r'x [\textmu m]')
> Nevermind: u'µm' works OK in the notebook, just not in GTKAgg. This is probably a local issue. I'll go with u'µm' -- typographic correctness triumphs.
> 
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