Implement geom_line with size mapping

[jankatins]

See this matplotlib bug for the details:

matplotlib/matplotlib#2658 (comment)

[jankatins]

Here we have some code which does the deed:
from http://nbviewer.ipython.org/github/brianckeegan/Bechdel/blob/master/Bechdel_test.ipynb cell 37

# Changing line widths from: http://stackoverflow.com/questions/19862011/matplotlib-change-linewidth-on-line-segments-using-list
coords = sorted(dict(avg_bechdel).items(),key=lambda x:x[0])
lines = [(start, end) for start, end in zip(coords[:-1], coords[1:])]
lines = LineCollection(lines, linewidths=dict(num_movies).values())
# Plot the figure
fig, ax = plt.subplots()
ax.add_collection(lines)
plt.autoscale()
plt.xlim((1912,2020))
plt.xlabel('Time',fontsize=18)
plt.ylabel('Avg. Bechdel score',fontsize=18)

[has2k1]

Good find, I'll try to add it to #266.

[jankatins]

There is also the possibility to use "fill_between". We would need to compute the "betweens", but that shouldn't be hard ([line[x]-size[x],line[x]+size[x] for x in xmapping])

See second example here:
http://www.randalolson.com/2014/06/28/how-to-make-beautiful-data-visualizations-in-python-with-matplotlib/

[has2k1]

When I played around with the first method (line collections), it could become very slow with increased points. On the other hand I have used "fill_between" to create a bar plot way faster than matplotlib does it.

A head on benchmark between the two is needed.