相对于echarts等基于JavaScript的图表库，matplotlib的交互能力相对较差。

在实际应用中，我们经常想使用十字光标来定位数据坐标，matplotlib内置提供支持。

官方示例

matplotlib提供了官方示例https://matplotlib.org/gallery/widgets/cursor.html

from matplotlib.widgets import Cursor

import numpy as np

import matplotlib.pyplot as plt

# Fixing random state for reproducibility

np.random.seed(19680801)

fig = plt.figure(figsize=(8, 6))

ax = fig.add_subplot(111, facecolor='#FFFFCC')

x, y = 4*(np.random.rand(2, 100) - .5)

ax.plot(x, y, 'o')

ax.set_xlim(-2, 2)

ax.set_ylim(-2, 2)

# Set useblit=True on most backends for enhanced performance.

cursor = Cursor(ax, useblit=True, color='red', linewidth=2)

plt.show()

原理

由源码可知，实现十字光标的关键在于widgets模块中的Cursor类。

class matplotlib.widgets.Cursor(ax, horizOn=True, vertOn=True, useblit=False, **lineprops)

ax：参数类型matplotlib.axes.Axes，即需要添加十字光标的子图。

horizOn：布尔值，是否显示十字光标中的横线，默认值为显示。

vertOn：布尔值，是否显示十字光标中的竖线，默认值为显示。

useblit：布尔值，是否使用优化模式，默认值为否，优化模式需要后端支持。

**lineprops：十字光标线形属性， 参见axhline函数支持的属性，https://matplotlib.org/api/_as_gen/matplotlib.axes.Axes.axhline.html#matplotlib.axes.Axes.axhline。

简化案例

光标改为灰色竖虚线，线宽为1。

from matplotlib.widgets import Cursor

import matplotlib.pyplot as plt

ax = plt.gca()

cursor = Cursor(ax, horizOn=False, vertOn= True, useblit=False, color='grey', linewidth=1,linestyle='--')

plt.show()

## Cursor类源码

class Cursor(AxesWidget):

"""

A crosshair cursor that spans the axes and moves with mouse cursor.

For the cursor to remain responsive you must keep a reference to it.

Parameters

----------

ax : `matplotlib.axes.Axes`

The `~.axes.Axes` to attach the cursor to.

horizOn : bool, default: True

Whether to draw the horizontal line.

vertOn : bool, default: True

Whether to draw the vertical line.

useblit : bool, default: False

Use blitting for faster drawing if supported by the backend.

Other Parameters

----------------

**lineprops

`.Line2D` properties that control the appearance of the lines.

See also `~.Axes.axhline`.

Examples

--------

See :doc:`/gallery/widgets/cursor`.

"""

def __init__(self, ax, horizOn=True, vertOn=True, useblit=False,

**lineprops):

AxesWidget.__init__(self, ax)

self.connect_event('motion_notify_event', self.onmove)

self.connect_event('draw_event', self.clear)

self.visible = True

self.horizOn = horizOn

self.vertOn = vertOn

self.useblit = useblit and self.canvas.supports_blit

if self.useblit:

lineprops['animated'] = True

self.lineh = ax.axhline(ax.get_ybound()[0], visible=False, **lineprops)

self.linev = ax.axvline(ax.get_xbound()[0], visible=False, **lineprops)

self.background = None

self.needclear = False

def clear(self, event):

"""Internal event handler to clear the cursor."""

if self.ignore(event):

return

if self.useblit:

self.background = self.canvas.copy_from_bbox(self.ax.bbox)

self.linev.set_visible(False)

self.lineh.set_visible(False)

def onmove(self, event):

"""Internal event handler to draw the cursor when the mouse moves."""

if self.ignore(event):

return

if not self.canvas.widgetlock.available(self):

return

if event.inaxes != self.ax:

self.linev.set_visible(False)

self.lineh.set_visible(False)

if self.needclear:

self.canvas.draw()

self.needclear = False

return

self.needclear = True

if not self.visible:

return

self.linev.set_xdata((event.xdata, event.xdata))

self.lineh.set_ydata((event.ydata, event.ydata))

self.linev.set_visible(self.visible and self.vertOn)

self.lineh.set_visible(self.visible and self.horizOn)

self._update()

def _update(self):

if self.useblit:

if self.background is not None:

self.canvas.restore_region(self.background)

self.ax.draw_artist(self.linev)

self.ax.draw_artist(self.lineh)

self.canvas.blit(self.ax.bbox)

else:

self.canvas.draw_idle()

return False

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