Question

In [1]:

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

%matplotlib inline

legend() 函数被用来添加图像的标签，其主要相关的属性有：

• legend entry - 一个 legend 包含一个或多个 entry，一个 entry 对应一个 key 和一个 label
• legend key - marker 的标记
• legend label - key 的说明
• legend handle - 一个 entry 在图上对应的对象

使用 legend

调用 legend() 会自动获取当前的 Axes 对象，并且得到这些 handles 和 labels，相当于：

handles, labels = ax.get_legend_handles_labels()
ax.legend(handles, labels)

我们可以在函数中指定 handles 的参数：

In [2]:

line_up, = plt.plot([1,2,3], label='Line 2')
line_down, = plt.plot([3,2,1], label='Line 1')
plt.legend(handles=[line_up, line_down])
plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/Yql34T8N3xceYtwk-a2aC15.png alt="">

可以将 labels 作为参数输入 legend 函数：

In [3]:

line_up, = plt.plot([1,2,3])
line_down, = plt.plot([3,2,1])
plt.legend([line_up, line_down], ['Line Up', 'Line Down'])
plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/0kL1rcctqxtxWxMr-wEstmT.png alt="">

产生特殊形状的 marker key

有时我们可以产生一些特殊形状的 marker：

块状：

In [4]:

import matplotlib.patches as mpatches

red_patch = mpatches.Patch(color='red', label='The red data')
plt.legend(handles=[red_patch])

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/ensPdvfjbPoJYmgf-IbVV2n.png alt="">

点线组合：

In [5]:

import matplotlib.lines as mlines
import matplotlib.pyplot as plt

blue_line = mlines.Line2D([], [], color='blue', marker='*',
                          markersize=15, label='Blue stars')
plt.legend(handles=[blue_line])

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/oc9LsKVHW39zfYTc-SfKGCP.png alt="">

指定 legend 的位置

bbox_to_anchor 关键词可以指定 legend 放置的位置，例如放到图像的右上角：

In [6]:

plt.plot([1,2,3], label="test1")
plt.plot([3,2,1], label="test2")
plt.legend(bbox_to_anchor=(1, 1),
           bbox_transform=plt.gcf().transFigure)

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/TE5OEsjii1rijKfi-W2ai93.png alt="">

更复杂的用法：

In [7]:

plt.subplot(211)
plt.plot([1,2,3], label="test1")
plt.plot([3,2,1], label="test2")
# Place a legend above this legend, expanding itself to
# fully use the given bounding box.
plt.legend(bbox_to_anchor=(0., 1.02, 1., .102), loc=3,
           ncol=2, mode="expand", borderaxespad=0.)

plt.subplot(223)
plt.plot([1,2,3], label="test1")
plt.plot([3,2,1], label="test2")
# Place a legend to the right of this smaller figure.
plt.legend(bbox_to_anchor=(1.05, 1), loc=2, borderaxespad=0.)

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/MChJKtmvWpFbRpSu-iwvGaJ.png alt="">

同一个 Axes 中的多个 legend

可以这样添加多个 legend：

In [8]:

line1, = plt.plot([1,2,3], label="Line 1", linestyle='--')
line2, = plt.plot([3,2,1], label="Line 2", linewidth=4)

# Create a legend for the first line.
first_legend = plt.legend(handles=[line1], loc=1)

# Add the legend manually to the current Axes.
ax = plt.gca().add_artist(first_legend)

# Create another legend for the second line.
plt.legend(handles=[line2], loc=4)

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/RGMnQasCqwCcNWqN-5DNrVg.png alt="">

其中 loc 参数可以取 0-10 或者 字符串，表示放置的位置：

loc string	loc code
'best'	0
'upper right'	1
'upper left'	2
'lower left'	3
'lower right'	4
'right'	5
'center left'	6
'center right'	7
'lower center'	8
'upper center'	9
'center'	10

更多用法

多个 handle 可以通过括号组合在一个 entry 中：

In [9]:

from numpy.random import randn

z = randn(10)

red_dot, = plt.plot(z, "ro", markersize=15)
# Put a white cross over some of the data.
white_cross, = plt.plot(z[:5], "w+", markeredgewidth=3, markersize=15)

plt.legend([red_dot, (red_dot, white_cross)], ["Attr A", "Attr A+B"])

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/BNlENLlxzDUZhg6k-Pzi76i.png alt="">

自定义 handle：

In [10]:

import matplotlib.pyplot as plt
import matplotlib.patches as mpatches

class AnyObject(object):
    pass

class AnyObjectHandler(object):
    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
        x0, y0 = handlebox.xdescent, handlebox.ydescent
        width, height = handlebox.width, handlebox.height
        patch = mpatches.Rectangle([x0, y0], width, height, facecolor='red',
                                   edgecolor='black', hatch='xx', lw=3,
                                   transform=handlebox.get_transform())
        handlebox.add_artist(patch)
        return patch

plt.legend([AnyObject()], ['My first handler'],
           handler_map={AnyObject: AnyObjectHandler()})

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/UJ7Ur1UBEEor8MVs-LHoZ1b.png alt="">

椭圆：

In [11]:

from matplotlib.legend_handler import HandlerPatch
import matplotlib.pyplot as plt
import matplotlib.patches as mpatches

class HandlerEllipse(HandlerPatch):
    def create_artists(self, legend, orig_handle,
                       xdescent, ydescent, width, height, fontsize, trans):
        center = 0.5 * width - 0.5 * xdescent, 0.5 * height - 0.5 * ydescent
        p = mpatches.Ellipse(xy=center, width=width + xdescent,
                             height=height + ydescent)
        self.update_prop(p, orig_handle, legend)
        p.set_transform(trans)
        return [p]

c = mpatches.Circle((0.5, 0.5), 0.25, facecolor="green",
                    edgecolor="red", linewidth=3)
plt.gca().add_patch(c)

plt.legend([c], ["An ellipse, not a rectangle"],
           handler_map={mpatches.Circle: HandlerEllipse()})

plt.show()

https://www.wenjiangs.com/wp-content/uploads/2022/docimg20/hj10yuJYhH2TuLIb-O0SSIf.png alt="">