hl_span with DatetimeIndex having issues

[cancan101]

This is the error:

----> 6 fig.hl_span("2014-4-14", "2014-4-14", color='red')

/usr/lib/python2.7/site-packages/ts_charting/span.pyc in hl_span_figure(self, *args, **kwargs)
     36     grapher = self.grapher
     37     kwargs['grapher'] = grapher
---> 38     return highlight_span(*args, **kwargs)
     39 
     40 charting.Figure.hl_span = hl_span_figure

/usr/lib/python2.7/site-packages/ts_charting/span.pyc in highlight_span(start, end, color, alpha, grapher)
     31         end = index.get_loc(end)
     32 
---> 33     grapher.ax.axvspan(start, end, color=color, alpha=alpha)
     34 
     35 def hl_span_figure(self, *args, **kwargs):

/usr/lib64/python2.7/site-packages/matplotlib/axes.pyc in axvspan(self, xmin, xmax, ymin, ymax, **kwargs)
   3647 
   3648         verts = [(xmin, ymin), (xmin, ymax), (xmax, ymax), (xmax, ymin)]
-> 3649         p = mpatches.Polygon(verts, **kwargs)
   3650         p.set_transform(trans)
   3651         self.add_patch(p)

/usr/lib64/python2.7/site-packages/matplotlib/patches.pyc in __init__(self, xy, closed, **kwargs)
    806         Patch.__init__(self, **kwargs)
    807         self._closed = closed
--> 808         self.set_xy(xy)
    809 
    810     def get_path(self):

/usr/lib64/python2.7/site-packages/matplotlib/patches.pyc in set_xy(self, xy)
    831             if len(xy) > 2 and (xy[0] == xy[-1]).all():
    832                 xy = xy[:-1]
--> 833         self._path = Path(xy, closed=self._closed)
    834 
    835     _get_xy = get_xy

/usr/lib64/python2.7/site-packages/matplotlib/path.pyc in __init__(self, vertices, codes, _interpolation_steps, closed, readonly)
    131             vertices = vertices.astype(np.float_).filled(np.nan)
    132         else:
--> 133             vertices = np.asarray(vertices, np.float_)
    134 
    135         if codes is not None:

/usr/lib64/python2.7/site-packages/numpy/core/numeric.pyc in asarray(a, dtype, order)
    458 
    459     """
--> 460     return array(a, dtype, copy=False, order=order)
    461 
    462 def asanyarray(a, dtype=None, order=None):

TypeError: float() argument must be a string or a number

I beleive this is caused by the assumption here:
https://github.com/dalejung/ts-charting/blob/master/ts_charting/span.py#L28 that get_loc returns an int.

Rather it returns a slice:

In [455]:
fig.grapher.index.get_loc("2014-4-14")

Out[455]:
slice(0, 421, None)

In [456]:
fig.grapher.index

Out[456]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2014-04-14 09:30:00, ..., 2014-04-21 11:54:55]
Length: 1840, Freq: None, Timezone: None

I would guess that if the return value of get_loc is a slice, then the start of the slice should be used for the start index and the stop should be used for the end:

start = index.get_loc(start).start
end= index.get_loc(end).stop