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:

So how to merge the markers into one label? also for the lines, for the lines, of course, u can realize it by not assigning label to the second line while using the same linetype, but for the markers, you can not, since they are of different shapes.

Answer 1

I think it's best to use a full legend - otherwise, how will your readers know the difference between the two models, or the two datasets? I would do it this way:

But, if you really want to do it your way, you can use a custom legend as shown in this guide. You'll need to create your own class, like they do, that defines the legend_artist method, which then adds squares and circles as appropriate. Here is the plot generated and the code used to generate it:

#!/usr/bin/env python
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches
import numpy as np


# ==================================
# Define the form of the function
# ==================================
def model(x, A=190, k=1):
    return A * np.exp(-k*x/50)

# ==================================
# How many data points are generated
# ==================================
num_samples = 15

# ==================================
# Create data for plots
# ==================================
x_model = np.linspace(0, 130, 200)

x_data1 = np.random.rand(num_samples) * 130
x_data1.sort()

x_data2 = np.random.rand(num_samples) * 130
x_data2.sort()

data1 = model(x_data1, k=1) * (1 + np.random.randn(num_samples) * 0.2)
data2 = model(x_data2, k=2) * (1 + np.random.randn(num_samples) * 0.15)

model1 = model(x_model, k=1)
model2 = model(x_model, k=2)

# ==================================
# Plot everything normally
# ==================================
fig = plt.figure()
ax = fig.add_subplot('111')
ax.plot(x_data1, data1, 'ok', markerfacecolor='none', label='Data (k=1)')
ax.plot(x_data2, data2, 'sk', markeredgecolor='0.5', markerfacecolor='0.5', label='Data (k=2)')
ax.plot(x_model, model1, '-k', label='Model (k=1)')
ax.plot(x_model, model2, '--k', label='Model (k=2)')

# ==================================
# Format plot
# ==================================
ax.set_xlabel('Distance from heated face($10^{-2}$ m)')
ax.set_ylabel('Temperature ($^\circ$C)')
ax.set_xlim((0, 130))
ax.set_title('Normal way to plot')
ax.legend()
fig.tight_layout()

plt.show()


# ==================================
# ==================================
# Do it again, but with custom
# legend
# ==================================
# ==================================
class AnyObject(object):
    pass


class data_handler(object):
    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
        scale = fontsize / 22
        x0, y0 = handlebox.xdescent, handlebox.ydescent
        width, height = handlebox.width, handlebox.height
        patch_sq = mpatches.Rectangle([x0, y0 + height/2 * (1 - scale) ], height * scale, height * scale, facecolor='0.5',
                edgecolor='0.5', transform=handlebox.get_transform())
        patch_circ = mpatches.Circle([x0 + width - height/2, y0 + height/2], height/2 * scale, facecolor='none',
                edgecolor='black', transform=handlebox.get_transform())

        handlebox.add_artist(patch_sq)
        handlebox.add_artist(patch_circ)
        return patch_sq

# ==================================
# Plot everything
# ==================================
fig = plt.figure()
ax = fig.add_subplot('111')
d1 = ax.plot(x_data1, data1, 'ok', markerfacecolor='none', label='Data (k=2)')
d2 = ax.plot(x_data2, data2, 'sk', markeredgecolor='0.5', markerfacecolor='0.5', label='Data (k=1)')
m1 = ax.plot(x_model, model1, '-k', label='Model (k=1)')
m2 = ax.plot(x_model, model2, '-k', label='Model (k=2)')

# ax.legend([d1], handler_map={ax.plot: data_handler()})
ax.legend([AnyObject(), m1[0]], ['Data', 'Model'], handler_map={AnyObject: data_handler()})

# ==================================
# Format plot
# ==================================
ax.set_xlabel('Distance from heated face($10^{-2}$ m)')
ax.set_ylabel('Temperature ($^\circ$C)')
ax.set_xlim((0, 130))
ax.set_title('Custom legend')
fig.tight_layout()

plt.show()

Answer 2

I also found this link very useful (code below), it's an easier way to handle this issue. It's basically using a list of legend handles to make one of the markers of the first handle invisible and overplot it with the marker of the second handle. This way, you have both markers next to each other with one label.

fig, ax = plt.subplots()
p1 = ax.scatter([0.1],[0.5],c='r',marker='s')
p2 = ax.scatter([0.3],[0.2],c='b',marker='o')
l = ax.legend([(p1,p2)],['points'],scatterpoints=2)

With the above code, a TupleHandler is used to create legend handles which simply overplot two handles (there are red squares behind the blue circles if you look carefylly. What you want to do is make the second marker of first handle and the first marker of the second handle invisible. Unfortunately, the TupleHandler is a rather recent addition and you need a special function to get all the handles. Otherwise, you can use the Legend.legendHandles attribute (it only show the first handle for the TupleHandler).

def get_handle_lists(l):
    """returns a list of lists of handles.
    """
    tree = l._legend_box.get_children()[1]

    for column in tree.get_children():
        for row in column.get_children():
            yield row.get_children()[0].get_children()

handles_list = list(get_handle_lists(l))
handles = handles_list[0] # handles is a list of two PathCollection.
                          # The first one is for red squares, and the second
                          # is for blue circles.
handles[0].set_facecolors(["r", "none"]) # for the fist
                   # PathCollection, make the
                   # second marker invisible by
                   # setting their facecolor and
                   # edgecolor to "none."
handles[0].set_edgecolors(["k", "none"])
handles[1].set_facecolors(["none", "b"])
handles[1].set_edgecolors(["none", "k"])
fig