matplotlib绘制鼠标的十字光标的实现(自定义方式，

matplotlib在widgets模块提供Cursor类用于支持十字光标的生成。另外官方还提供了自定义十字光标的实例。

widgets模块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

自定义十字光标实现

简易十字光标实现

首先在 Cursor类的构造方法__init__中，构造了十字光标的横线、竖线和坐标显示；然后在on_mouse_move方法中，根据事件数据更新横竖线和坐标显示，最后在调用时，通过mpl_connect方法绑定on_mouse_move方法和鼠标移动事件'motion_notify_event'。

import matplotlib.pyplot as plt
import numpy as np


class Cursor:
  """
  A cross hair cursor.
  """
  def __init__(self, ax):
    self.ax = ax
    self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
    self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
    # text location in axes coordinates
    self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)

  def set_cross_hair_visible(self, visible):
    need_redraw = self.horizontal_line.get_visible() != visible
    self.horizontal_line.set_visible(visible)
    self.vertical_line.set_visible(visible)
    self.text.set_visible(visible)
    return need_redraw

  def on_mouse_move(self, event):
    if not event.inaxes:
      need_redraw = self.set_cross_hair_visible(False)
      if need_redraw:
        self.ax.figure.canvas.draw()
    else:
      self.set_cross_hair_visible(True)
      x, y = event.xdata, event.ydata
      # update the line positions
      self.horizontal_line.set_ydata(y)
      self.vertical_line.set_xdata(x)
      self.text.set_text('x=%1.2f, y=%1.2f' % (x, y))
      self.ax.figure.canvas.draw()


x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)

fig, ax = plt.subplots()
ax.set_title('Simple cursor')
ax.plot(x, y, 'o')
cursor = Cursor(ax)
#关键部分，绑定鼠标移动事件处理
fig.canvas.mpl_connect('motion_notify_event', cursor.on_mouse_move)
plt.show()

优化十字光标实现

在简易实现中，每次鼠标移动时，都会重绘整个图像，这样效率比较低。
在优化实现中，每次鼠标移动时，只重绘光标和坐标显示，背景图像不再重绘。

import matplotlib.pyplot as plt
import numpy as np


class BlittedCursor:
  """
  A cross hair cursor using blitting for faster redraw.
  """
  def __init__(self, ax):
    self.ax = ax
    self.background = None
    self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
    self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
    # text location in axes coordinates
    self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)
    self._creating_background = False
    ax.figure.canvas.mpl_connect('draw_event', self.on_draw)

  def on_draw(self, event):
    self.create_new_background()

  def set_cross_hair_visible(self, visible):
    need_redraw = self.horizontal_line.get_visible() != visible
    self.horizontal_line.set_visible(visible)
    self.vertical_line.set_visible(visible)
    self.text.set_visible(visible)
    return need_redraw

  def create_new_background(self):
    if self._creating_background:
      # discard calls triggered from within this function
      return
    self._creating_background = True
    self.set_cross_hair_visible(False)
    self.ax.figure.canvas.draw()
    self.background = self.ax.figure.canvas.copy_from_bbox(self.ax.bbox)
    self.set_cross_hair_visible(True)
    self._creating_background = False

  def on_mouse_move(self, event):
    if self.background is None:
      self.create_new_background()
    if not event.inaxes:
      need_redraw = self.set_cross_hair_visible(False)
      if need_redraw:
        self.ax.figure.canvas.restore_region(self.background)
        self.ax.figure.canvas.blit(self.ax.bbox)
    else:
      self.set_cross_hair_visible(True)
      # update the line positions
      x, y = event.xdata, event.ydata
      self.horizontal_line.set_ydata(y)
      self.vertical_line.set_xdata(x)
      self.text.set_text('x=%1.2f, y=%1.2f' % (x, y))

      self.ax.figure.canvas.restore_region(self.background)
      self.ax.draw_artist(self.horizontal_line)
      self.ax.draw_artist(self.vertical_line)
      self.ax.draw_artist(self.text)
      self.ax.figure.canvas.blit(self.ax.bbox)


x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)

fig, ax = plt.subplots()
ax.set_title('Blitted cursor')
ax.plot(x, y, 'o')
blitted_cursor = BlittedCursor(ax)
fig.canvas.mpl_connect('motion_notify_event', blitted_cursor.on_mouse_move)
plt.show()

捕捉数据十字光标实现

在前面的两种实现中，鼠标十字光标可以随意移动。在本实现中，十字光标只会出现在离鼠标x坐标最近的数据点上。

import matplotlib.pyplot as plt
import numpy as np


class SnappingCursor:
  """
  A cross hair cursor that snaps to the data point of a line, which is
  closest to the *x* position of the cursor.

  For simplicity, this assumes that *x* values of the data are sorted.
  """
  def __init__(self, ax, line):
    self.ax = ax
    self.horizontal_line = ax.axhline(color='k', lw=0.8, ls='--')
    self.vertical_line = ax.axvline(color='k', lw=0.8, ls='--')
    self.x, self.y = line.get_data()
    self._last_index = None
    # text location in axes coords
    self.text = ax.text(0.72, 0.9, '', transform=ax.transAxes)

  def set_cross_hair_visible(self, visible):
    need_redraw = self.horizontal_line.get_visible() != visible
    self.horizontal_line.set_visible(visible)
    self.vertical_line.set_visible(visible)
    self.text.set_visible(visible)
    return need_redraw

  def on_mouse_move(self, event):
    if not event.inaxes:
      self._last_index = None
      need_redraw = self.set_cross_hair_visible(False)
      if need_redraw:
        self.ax.figure.canvas.draw()
    else:
      self.set_cross_hair_visible(True)
      x, y = event.xdata, event.ydata
      index = min(np.searchsorted(self.x, x), len(self.x) - 1)
      if index == self._last_index:
        return # still on the same data point. Nothing to do.
      self._last_index = index
      x = self.x[index]
      y = self.y[index]
      # update the line positions
      self.horizontal_line.set_ydata(y)
      self.vertical_line.set_xdata(x)
      self.text.set_text('x=%1.2f, y=%1.2f' % (x, y))
      self.ax.figure.canvas.draw()


x = np.arange(0, 1, 0.01)
y = np.sin(2 * 2 * np.pi * x)

fig, ax = plt.subplots()
ax.set_title('Snapping cursor')
line, = ax.plot(x, y, 'o')
snap_cursor = SnappingCursor(ax, line)
fig.canvas.mpl_connect('motion_notify_event', snap_cursor.on_mouse_move)
plt.show()

参考资料

https://www.matplotlib.org.cn/gallery/misc/cursor_demo_sgskip.html

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