06.07 标签

calendar May 5, 2026 · 3 分钟阅读 · Python Matplotlib 图例  ·
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标签

1import numpy as np
2import matplotlib as mpl
3import matplotlib.pyplot as plt
4
5%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：

1line_up, = plt.plot([1,2,3], label='Line 2')
2line_down, = plt.plot([3,2,1], label='Line 1')
3plt.legend(handles=[line_up, line_down])
4plt.show()

png

可以将 labels 作为参数输入 legend 函数：

1line_up, = plt.plot([1,2,3])
2line_down, = plt.plot([3,2,1])
3plt.legend([line_up, line_down], ['Line Up', 'Line Down'])
4plt.show()

png

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：

1import matplotlib.patches as mpatches
2
3red_patch = mpatches.Patch(color='red', label='The red data')
4plt.legend(handles=[red_patch])
5
6plt.show()

png

点线组合：

1import matplotlib.lines as mlines
2import matplotlib.pyplot as plt
3
4blue_line = mlines.Line2D([], [], color='blue', marker='*',
5                          markersize=15, label='Blue stars')
6plt.legend(handles=[blue_line])
7
8plt.show()

png

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：

1plt.plot([1,2,3], label="test1")
2plt.plot([3,2,1], label="test2")
3plt.legend(bbox_to_anchor=(1, 1),
4           bbox_transform=plt.gcf().transFigure)
5
6plt.show()

png

更复杂的用法：

 1plt.subplot(211)
 2plt.plot([1,2,3], label="test1")
 3plt.plot([3,2,1], label="test2")
 4# Place a legend above this legend, expanding itself to
 5# fully use the given bounding box.
 6plt.legend(bbox_to_anchor=(0., 1.02, 1., .102), loc=3,
 7           ncol=2, mode="expand", borderaxespad=0.)
 8
 9plt.subplot(223)
10plt.plot([1,2,3], label="test1")
11plt.plot([3,2,1], label="test2")
12# Place a legend to the right of this smaller figure.
13plt.legend(bbox_to_anchor=(1.05, 1), loc=2, borderaxespad=0.)
14
15plt.show()

png

同一个 Axes 中的多个 legend

可以这样添加多个 legend：

 1line1, = plt.plot([1,2,3], label="Line 1", linestyle='--')
 2line2, = plt.plot([3,2,1], label="Line 2", linewidth=4)
 3
 4# Create a legend for the first line.
 5first_legend = plt.legend(handles=[line1], loc=1)
 6
 7# Add the legend manually to the current Axes.
 8ax = plt.gca().add_artist(first_legend)
 9
10# Create another legend for the second line.
11plt.legend(handles=[line2], loc=4)
12
13plt.show()

png

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：

 1from numpy.random import randn
 2
 3z = randn(10)
 4
 5red_dot, = plt.plot(z, "ro", markersize=15)
 6# Put a white cross over some of the data.
 7white_cross, = plt.plot(z[:5], "w+", markeredgewidth=3, markersize=15)
 8
 9plt.legend([red_dot, (red_dot, white_cross)], ["Attr A", "Attr A+B"])
10
11plt.show()

png

自定义 handle：

 1import matplotlib.pyplot as plt
 2import matplotlib.patches as mpatches
 3
 4class AnyObject(object):
 5    pass
 6
 7class AnyObjectHandler(object):
 8    def legend_artist(self, legend, orig_handle, fontsize, handlebox):
 9        x0, y0 = handlebox.xdescent, handlebox.ydescent
10        width, height = handlebox.width, handlebox.height
11        patch = mpatches.Rectangle([x0, y0], width, height, facecolor='red',
12                                   edgecolor='black', hatch='xx', lw=3,
13                                   transform=handlebox.get_transform())
14        handlebox.add_artist(patch)
15        return patch
16
17plt.legend([AnyObject()], ['My first handler'],
18           handler_map={AnyObject: AnyObjectHandler()})
19
20plt.show()

png

椭圆：

 1from matplotlib.legend_handler import HandlerPatch
 2import matplotlib.pyplot as plt
 3import matplotlib.patches as mpatches
 4
 5
 6class HandlerEllipse(HandlerPatch):
 7    def create_artists(self, legend, orig_handle,
 8                       xdescent, ydescent, width, height, fontsize, trans):
 9        center = 0.5 * width - 0.5 * xdescent, 0.5 * height - 0.5 * ydescent
10        p = mpatches.Ellipse(xy=center, width=width + xdescent,
11                             height=height + ydescent)
12        self.update_prop(p, orig_handle, legend)
13        p.set_transform(trans)
14        return [p]
15
16
17c = mpatches.Circle((0.5, 0.5), 0.25, facecolor="green",
18                    edgecolor="red", linewidth=3)
19plt.gca().add_patch(c)
20
21plt.legend([c], ["An ellipse, not a rectangle"],
22           handler_map={mpatches.Circle: HandlerEllipse()})
23
24plt.show()

png