annotate with two different colors based on threshold

I added the capability to annotate an image with features of two different colors. I found this useful for playing around with cutoff thresholds. e.g. see how many features you miss if you increase the minimum size.
My fault. This is what I get for late-night merging.

On Wednesday, July 9, 2014, Nathan Keim <notifications@github.com> wrote:

> In trackpy/plots.py:
>
> >      Returns
> >      ------
> >      axes
> >      """
> >      import matplotlib.pyplot as plt
> > +    from itertools import tee, izip
> > +    from collections import Iterable
> > +
> > +    # https://docs.python.org/2/library/itertools.html
> > +    def pairwise(iterable):
> > +        "s -> (s0,s1), (s1,s2), (s2, s3), ..."
> > +        a, b = tee(iterable)
> > +        next(b, None)
> > +        return izip(a, b)
> > +
> > +    if color is None:
> > +        color = 'g'
> > +    if not (split_thresh, Iterable):
>
> I know the train's already left the station, but I don't see how this test
> wouldn't always evaluate to True. Did you mean to write if not
> isinstance(split_thresh, Iterable)?
>
> In any case, a string also counts as iterable, so there has to be another
> way.
>
> Sorry I've been doing nothing but writing comments. I'll be back on my
> development computer by tomorrow, I promise :).
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/soft-matter/trackpy/pull/104/files#r14724711>.
>