How to make two markers share the same label in the legend using matplotlib?

Question

What I want is like this:

What I get is this:

Answer 1

Here is a new solution that will plot any collection of markers with the same label. I have not figured out how to make it work with markers from a line plot, but you can probably do a scatter plot on top of a line plot if you need to.

from matplotlib import pyplot as plt
import matplotlib.collections as mcol
import matplotlib.transforms as mtransforms
import numpy as np
from matplotlib.legend_handler import HandlerPathCollection
from matplotlib import cm


class HandlerMultiPathCollection(HandlerPathCollection):
    """
    Handler for PathCollections, which are used by scatter
    """
    def create_collection(self, orig_handle, sizes, offsets, transOffset):
        p = type(orig_handle)(orig_handle.get_paths(), sizes=sizes,
                              offsets=offsets,
                              transOffset=transOffset,
                              )
        return p

fig, ax = plt.subplots()
#make some data to plot
x = np.arange(0, 100, 10)
models = [.05 * x, 8 * np.exp(- .1 * x), np.log(x + 1), .01 * x]
tests = [model + np.random.rand(len(model)) - .5 for model in models]
#make colors and markers
colors = cm.brg(np.linspace(0, 1, len(models)))
markers = ['o', 'D', '*', 's']
markersize = 50
plots = []
#plot points and lines
for i in xrange(len(models)):
    line, = plt.plot(x, models[i], linestyle = 'dashed', color = 'black', label = 'Model')
    plot = plt.scatter(x, tests[i], c = colors[i], s = markersize, marker = markers[i])
    plots.append(plot)

#get attributes
paths = []
sizes = []
facecolors = []
edgecolors = []
for plot in plots:
    paths.append(plot.get_paths()[0])
    sizes.append(plot.get_sizes()[0])
    edgecolors.append(plot.get_edgecolors()[0])
    facecolors.append(plot.get_facecolors()[0])

#make proxy artist out of a collection of markers
PC = mcol.PathCollection(paths, sizes, transOffset = ax.transData, facecolors = colors, edgecolors = edgecolors)
PC.set_transform(mtransforms.IdentityTransform())
plt.legend([PC, line], ['Test', 'Model'], handler_map = {type(PC) : HandlerMultiPathCollection()}, scatterpoints = len(paths), scatteryoffsets = [.5], handlelength = len(paths))
plt.show()

I have a solution for you if you're willing to use all circles for markers and differentiate by color only. You can use a circle collection to represent the markers, and then have a legend label for the collection as a whole.

Example code:

import matplotlib.pyplot as plt
import matplotlib.collections as collections
from matplotlib import cm
import numpy as np

#make some data to plot
x = np.arange(0, 100, 10)
models = [.05 * x, 8 * np.exp(- .1 * x), np.log(x + 1), .01 * x]
tests = [model + np.random.rand(len(model)) - .5 for model in models]
#make colors
colors = cm.brg(np.linspace(0, 1, len(models)))
markersize = 50
#plot points and lines
for i in xrange(len(models)):
    line, = plt.plot(x, models[i], linestyle = 'dashed', color = 'black', label = 'Model')
    plt.scatter(x, tests[i], c = colors[i], s = markersize)
#create collection of circles corresponding to markers
circles = collections.CircleCollection([markersize] * len(models), facecolor = colors)
#make the legend -- scatterpoints needs to be the same as the number 
#of markers so that all the markers show up in the legend
plt.legend([circles, line], ['Test', 'Model'], scatterpoints = len(models), scatteryoffsets = [.5], handlelength = len(models))
plt.show()