CODEWITHSUNDEEP
pythonmatplotliblegend
Andrea Zonca
Andrea Zonca

Reputation: 8783

increase the linewidth of the legend lines in matplotlib

I know that if I change the linewidth of a line, that is automatically updated in the legend. However I would like to just change the legend linewidth without affecting the plot.

Upvotes: 91

Views: 71517

Answers (4)

jdhao
jdhao

Reputation: 28487

@Brendan Wood's method uses the api provided by pyplot. In matplotlib, the object oriented style using axes is prefered. The following is how you can achieve this using axes method.

import numpy as np
import matplotlib.pyplot as plt

# make some data
x = np.linspace(0, 2*np.pi)
y1 = np.sin(x)
y2 = np.cos(x)

fig, ax = plt.subplots()
ax.plot(x, y1, linewidth=1.0, label='sin(x)')
ax.plot(x, y2, linewidth=1.0, label='cos(x)')

# get the legend object
leg = ax.legend()

# change the line width for the legend
for line in leg.get_lines():
    line.set_linewidth(4.0)

plt.show()

The produced plot is shown below, enter image description here

Upvotes: 42

ImportanceOfBeingErnest
ImportanceOfBeingErnest

Reputation: 339660

By default, the legend contains the lines themselves. Therefore changing the linewidth of the lines in the canvas will also change the lines in the legend (and vice versa, as they are essentially the same object).

A possible solution is to use a copy of the artist from the canvas and change only the copy's linewidth.

import numpy as np
import matplotlib.pyplot as plt
import copy

x = np.linspace(0, 2*np.pi)
y1 = np.sin(x)
y2 = np.cos(x)
fig = plt.figure()
ax  = fig.add_subplot(111)
ax.plot(x, y1, c='b', label='y1',linewidth=1.0)
ax.plot(x, y2, c='r', label='y2')

# obtain the handles and labels from the figure
handles, labels = ax.get_legend_handles_labels()
# copy the handles
handles = [copy.copy(ha) for ha in handles ]
# set the linewidths to the copies
[ha.set_linewidth(7) for ha in handles ]
# put the copies into the legend
leg = plt.legend(handles=handles, labels=labels)

plt.savefig('leg_example')
plt.show()

enter image description here

A different option would be to use a handler_map and an updating function. This is somehow automatic, specifying the handler map would automatically make any line in the legend 7 points wide.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.legend_handler import HandlerLine2D

x = np.linspace(0, 2*np.pi)
y1 = np.sin(x)
y2 = np.cos(x)
fig = plt.figure()
ax  = fig.add_subplot(111)
ax.plot(x, y1, c='b', label='y1',linewidth=1.0)
ax.plot(x, y2, c='r', label='y2')

linewidth=7
def update(handle, orig):
    handle.update_from(orig)
    handle.set_linewidth(7)

plt.legend(handler_map={plt.Line2D : HandlerLine2D(update_func=update)})

plt.show()

The result is the same as above.

Upvotes: 7

Brendan Wood
Brendan Wood

Reputation: 6450

Here's a simple example of how to do it:

import numpy as np
import matplotlib.pyplot as plt

# make some data
x = np.linspace(0, 2*np.pi)
y1 = np.sin(x)
y2 = np.cos(x)

# plot sin(x) and cos(x)
p1 = plt.plot(x, y1, 'b-', linewidth=1.0)
p2 = plt.plot(x, y2, 'r-', linewidth=1.0)

# make a legend for both plots
leg = plt.legend([p1, p2], ['sin(x)', 'cos(x)'], loc=1)

# set the linewidth of each legend object
for legobj in leg.legendHandles:
    legobj.set_linewidth(2.0)

plt.show()

Upvotes: 108

David Zwicker
David Zwicker

Reputation: 24328

If you want to change all lines in a plot, it might be useful to define your own legend handler:

import matplotlib.pyplot as plt
from matplotlib import legend_handler
from matplotlib.lines import Line2D
import numpy as np

class MyHandlerLine2D(legend_handler.HandlerLine2D):
    def create_artists(self, legend, orig_handle,
                       xdescent, ydescent, width, height, fontsize,
                       trans):

        xdata, xdata_marker = self.get_xdata(legend, xdescent, ydescent,
                                             width, height, fontsize)

        ydata = ((height-ydescent)/2.)*np.ones(xdata.shape, float)
        legline = Line2D(xdata, ydata)

        self.update_prop(legline, orig_handle, legend)
        #legline.update_from(orig_handle)
        #legend._set_artist_props(legline) # after update
        #legline.set_clip_box(None)
        #legline.set_clip_path(None)
        legline.set_drawstyle('default')
        legline.set_marker("")
        legline.set_linewidth(10)


        legline_marker = Line2D(xdata_marker, ydata[:len(xdata_marker)])
        self.update_prop(legline_marker, orig_handle, legend)
        #legline_marker.update_from(orig_handle)
        #legend._set_artist_props(legline_marker)
        #legline_marker.set_clip_box(None)
        #legline_marker.set_clip_path(None)
        legline_marker.set_linestyle('None')
        if legend.markerscale != 1:
            newsz = legline_marker.get_markersize()*legend.markerscale
            legline_marker.set_markersize(newsz)
        # we don't want to add this to the return list because
        # the texts and handles are assumed to be in one-to-one
        # correpondence.
        legline._legmarker = legline_marker

        return [legline, legline_marker]


plt.plot( [0, 1], [0, 1], '-r', lw=1, label='Line' )
plt.legend(handler_map={Line2D:MyHandlerLine2D()})

plt.show()

Upvotes: 4

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