matplotlib绘制鼠标的十字光标（内置方式）
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