Single legend item with two lines

Solution 1:

In the matplotlib legend guide there is a chapter about custom legend handlers. You could adapt it to your needs, e.g. like this:

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.legend_handler import HandlerBase


class AnyObjectHandler(HandlerBase):
    def create_artists(self, legend, orig_handle,
                       x0, y0, width, height, fontsize, trans):
        l1 = plt.Line2D([x0,y0+width], [0.7*height,0.7*height], 
                                                linestyle='--', color='k')
        l2 = plt.Line2D([x0,y0+width], [0.3*height,0.3*height], color='r')
        return [l1, l2]


x = np.linspace(0, 3)
fig, axL = plt.subplots(figsize=(4,3))
axR = axL.twinx()

axL.plot(x, np.sin(x), color='k', linestyle='--')
axR.plot(x, 100*np.cos(x), color='r')

axL.set_ylabel('sin(x)', color='k')
axR.set_ylabel('100 cos(x)', color='r')
axR.tick_params('y', colors='r')

plt.legend([object], ['label'],
           handler_map={object: AnyObjectHandler()})

plt.show()

In order to have multiple such entries, one can supply some tuple of parameters that the Handler then uses to draw the legend.

import matplotlib.pyplot as plt
import numpy as np
from matplotlib.legend_handler import HandlerBase


class AnyObjectHandler(HandlerBase):
    def create_artists(self, legend, orig_handle,
                       x0, y0, width, height, fontsize, trans):
        l1 = plt.Line2D([x0,y0+width], [0.7*height,0.7*height],
                           linestyle=orig_handle[1], color='k')
        l2 = plt.Line2D([x0,y0+width], [0.3*height,0.3*height], 
                           color=orig_handle[0])
        return [l1, l2]


x = np.linspace(0, 3)
fig, axL = plt.subplots(figsize=(4,3))
axR = axL.twinx()

axL.plot(x, np.sin(x), color='k', linestyle='--')
axR.plot(x, 100*np.cos(x), color='r')

axL.plot(x, .3*np.sin(x), color='k', linestyle=':')
axR.plot(x, 20*np.cos(x), color='limegreen')

axL.set_ylabel('sin(x)', color='k')
axR.set_ylabel('100 cos(x)', color='r')
axR.tick_params('y', colors='r')

plt.legend([("r","--"), ("limegreen",":")], ['label', "label2"],
           handler_map={tuple: AnyObjectHandler()})

plt.show()

Solution 2:

Here is an inadequate solution I have come to after playing around. I post it in case it helps others, but would prefer to do it properly. It uses two columns and the label of the first as a '/' or one could equally use "and".

import matplotlib.pyplot as plt

x = np.linspace(0, 3)
fig, axL = plt.subplots()
axR = axL.twinx()

axL.plot(x, np.sin(x), color='k', label='/')
axR.plot(x, 100*np.cos(x), color='r', label='label')

axL.set_ylabel('sin(x)', color='k')
axR.set_ylabel('100 cos(x)', color='r')
axR.tick_params('y', colors='r')

handlesL, labelsL = axL.get_legend_handles_labels()
handlesR, labelsR = axR.get_legend_handles_labels()
handles = handlesL + handlesR
labels = labelsL + labelsR
axR.legend(handles, labels, loc='lower center', ncol=2, fontsize=16,
           handletextpad=0.4, columnspacing=0.4)