即使调整图形大小，如何在图例和轴之间获得恒定距离？

Question

当使用bbox_to_anchor 将图例放置在坐标轴之外时，如this 答案中所示，调整图形大小时坐标轴和图例之间的空间会发生变化。对于静态导出的图，这很好；您可以简单地调整数字，直到正确为止。但是对于您可能想要调整大小的交互式绘图，这是一个问题。从这个例子可以看出：

import numpy as np
from matplotlib import pyplot as plt

x = np.arange(5)
y = np.random.randn(5)

fig, ax = plt.subplots(tight_layout=True)
ax.plot(x, y, label='data1')
ax.plot(x, y-1, label='data2')
legend = ax.legend(loc='upper center', bbox_to_anchor=(0.5, -0.05), ncol=2)
plt.show()

结果：

即使调整图形大小，如何确保图例与轴的距离相同？

Answer 1

图例与边界框边缘的距离由borderaxespad 参数设置。 borderaxespad 以字体大小的倍数为单位 - 使其自动独立于轴大小。 所以在这种情况下，

import matplotlib.pyplot as plt
import numpy as np
x = np.arange(5)
y = np.random.randn(5)

fig, ax = plt.subplots(constrained_layout=True)
ax.plot(x, y, label='data1')
ax.plot(x, y-1, label='data2')
legend = ax.legend(loc="upper center", bbox_to_anchor=(0.5,0), borderaxespad=2)

plt.show()

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Place title at the bottom of the figure of an axes? 中提出了关于在坐标轴下方以恒定距离显示标题的类似问题

Answer 2

您可以使用画布的调整大小事件在每次更新时更新bbox_to_anchor 中的值。要计算新值，您可以使用轴变换的逆 (Bbox.inverse_transformed(ax.transAxes))，它将屏幕坐标（以像素为单位）转换为通常在 bbox_to_anchor 中使用的轴坐标。

这是一个支持将图例放置在轴的所有四个边上的示例：

import numpy as np
from matplotlib import pyplot as plt
from matplotlib.transforms import Bbox


class FixedOutsideLegend:
    """A legend placed at a fixed offset (in pixels) from the axes."""

    def __init__(self, ax, location, pixel_offset, **kwargs):
        self._pixel_offset = pixel_offset

        self.location = location
        if location == 'right':
            self._loc = 'center left'
        elif location == 'left':
            self._loc = 'center right'
        elif location == 'upper':
            self._loc = 'lower center'
        elif location == 'lower':
            self._loc = 'upper center'
        else:
            raise ValueError('Unknown location: {}'.format(location))

        self.legend = ax.legend(
            loc=self._loc, bbox_to_anchor=self._get_bbox_to_anchor(), **kwargs)
        ax.figure.canvas.mpl_connect('resize_event', self.on_resize)

    def on_resize(self, event):
        self.legend.set_bbox_to_anchor(self._get_bbox_to_anchor())

    def _get_bbox_to_anchor(self):
        """
        Find the lengths in axes units that correspond to the specified
        pixel_offset.
        """
        screen_bbox = Bbox.from_bounds(
            0, 0, self._pixel_offset, self._pixel_offset)
        try:
            ax_bbox = screen_bbox.inverse_transformed(ax.transAxes)
        except np.linagl.LinAlgError:
            ax_width = 0
            ax_height = 0
        else:
            ax_width = ax_bbox.width
            ax_height = ax_bbox.height

        if self.location == 'right':
            return (1 + ax_width, 0.5)
        elif self.location == 'left':
            return (-ax_width, 0.5)
        elif self.location == 'upper':
            return (0.5, 1 + ax_height)
        elif self.location == 'lower':
            return (0.5, -ax_height)


x = np.arange(5)
y = np.random.randn(5)

fig, ax = plt.subplots(tight_layout=True)
ax.plot(x, y, label='data1')
ax.plot(x, y-1, label='data2')
legend = FixedOutsideLegend(ax, 'lower', 20, ncol=2)
plt.show()

结果：