Make rick.chart.line_chart's baseline param optional

[chmnata]

Function line_chart(data, ylab, xlab, **kwargs) currently is not taking baseline parameter as optional.

Documentation and reproducible example here

Error returned when using line_chart(data, ylab, xlab) without baseline=total[count]

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-293-3d92fba8004c> in <module>
     24 total2=pandasql.read_sql(query, con)
     25 
---> 26 fig, ax, props = rick.charts.line_chart(total['count'], 'Trips', 'Time')

/etc/jupyterhub/.venv/src/rick/charts/rick.py in line_chart(data, ylab, xlab, **kwargs)
    279         fig, ax =plt.subplots()
    280         ax.plot(data ,linewidth=3, color = colour.purple)
--> 281         ax.plot(baseline ,linewidth=3, color = colour.grey)
    282 
    283         plt.grid()

/etc/jupyterhub/.venv/lib/python3.5/site-packages/matplotlib/__init__.py in inner(ax, data, *args, **kwargs)
   1808                         "the Matplotlib list!)" % (label_namer, func.__name__),
   1809                         RuntimeWarning, stacklevel=2)
-> 1810             return func(ax, *args, **kwargs)
   1811 
   1812         inner.__doc__ = _add_data_doc(inner.__doc__,

/etc/jupyterhub/.venv/lib/python3.5/site-packages/matplotlib/axes/_axes.py in plot(self, scalex, scaley, *args, **kwargs)
   1609         kwargs = cbook.normalize_kwargs(kwargs, mlines.Line2D._alias_map)
   1610 
-> 1611         for line in self._get_lines(*args, **kwargs):
   1612             self.add_line(line)
   1613             lines.append(line)

/etc/jupyterhub/.venv/lib/python3.5/site-packages/matplotlib/axes/_base.py in _grab_next_args(self, *args, **kwargs)
    391                 this += args[0],
    392                 args = args[1:]
--> 393             yield from self._plot_args(this, kwargs)
    394 
    395 

/etc/jupyterhub/.venv/lib/python3.5/site-packages/matplotlib/axes/_base.py in _plot_args(self, tup, kwargs)
    351         # downstream.
    352         if any(v is None for v in tup):
--> 353             raise ValueError("x, y, and format string must not be None")
    354 
    355         kw = {}

ValueError: x, y, and format string must not be None