Subtitles within Matplotlib legend

Question

I am doing some plotting with matplotlib, and I have a legend which tells the viewer which sensors the points were recorded with. There are multiple sensors of multiple types, and I'd like to have subtitles within the legend to tell the viewer what kind of sensor each group is. I have a working solution, but it's a bit of a hack, shown here:

When the legend is created, it accepts two important arguments: a list of legend markers, and a list of legend labels. My current solution is to set the subtitle marker to be a white box with a white outline, and have the subtitle lable be surrounded by two newline characters. It looks alright, but it would look much more professional if the subtitles were not indented. Two workarounds I have tried have been setting the subtitle's marker to None, and also setting the subtitle's marker to the desired subtitle string and its label to an empty string. Neither have worked. Does anyone have any experience with this? Thanks so much.

Answer 1

The best I could come up with is to make a custom handler for a string.

import matplotlib.pyplot as plt
import matplotlib.text as mtext


class LegendTitle(object):
    def __init__(self, text_props=None):
        self.text_props = text_props or {}
        super(LegendTitle, self).__init__()

    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
        x0, y0 = handlebox.xdescent, handlebox.ydescent
        title = mtext.Text(x0, y0, r'\underline{' + orig_handle + '}', usetex=True, **self.text_props)
        handlebox.add_artist(title)
        return title


[line1] = plt.plot(range(10))
[line2] = plt.plot(range(10, 0, -1), 'o', color='red')
plt.legend(['Title 1', line1, 'Title 2', line2], ['', 'Line 1', '', 'Line 2'],
           handler_map={basestring: LegendTitle({'fontsize': 18})})

plt.show()

I based this on the example in http://matplotlib.org/users/legend_guide.html.