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/gallery/widgets/cursor.html
from matplotlib.widgets import Cursorimport numpy as npimport matplotlib.pyplot as plt# Fixing random state for reproducibilitynp.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，/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 Cursorimport matplotlib.pyplot as pltax = 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: TrueWhether to draw the horizontal line.vertOn : bool, default: TrueWhether to draw the vertical line.useblit : bool, default: FalseUse 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 = Trueself.horizOn = horizOnself.vertOn = vertOnself.useblit = useblit and self.canvas.supports_blitif self.useblit:lineprops['animated'] = Trueself.lineh = ax.axhline(ax.get_ybound()[0], visible=False, **lineprops)self.linev = ax.axvline(ax.get_xbound()[0], visible=False, **lineprops)self.background = Noneself.needclear = Falsedef clear(self, event):"""Internal event handler to clear the cursor."""if self.ignore(event):returnif 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):returnif not self.canvas.widgetlock.available(self):returnif event.inaxes != self.ax:self.linev.set_visible(False)self.lineh.set_visible(False)if self.needclear:self.canvas.draw()self.needclear = Falsereturnself.needclear = Trueif not self.visible:returnself.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
