matplotlib在widgets模組提供Cursor類用於支援十字游標的生成。另外官方還提供了自定義十字游標的例項。

widgets模組Cursor類原始碼

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],**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

自定義十字游標實現

簡易十字游標實現

首先在 Cursor類的構造方法__init__中，構造了十字游標的橫線、豎線和座標顯示；然後在on_mouse_move方法中，根據事件資料更新橫豎線和座標顯示，最後在呼叫時，通過mpl_connect方法繫結on_mouse_move方法和滑鼠移動事件'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',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)
#關鍵部分，繫結滑鼠移動事件處理
fig.canvas.mpl_connect('motion_notify_event',cursor.on_mouse_move)
plt.show()

優化十字游標實現

在簡易實現中，每次滑鼠移動時，都會重繪整個影象，這樣效率比較低。
在優化實現中，每次滑鼠移動時，只重繪游標和座標顯示，背景影象不再重繪。

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',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,event.ydata
      self.horizontal_line.set_ydata(y)
      self.vertical_line.set_xdata(x)
      self.text.set_text('x=%1.2f,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,ax = plt.subplots()
ax.set_title('Blitted cursor')
ax.plot(x,'o')
blitted_cursor = BlittedCursor(ax)
fig.canvas.mpl_connect('motion_notify_event',blitted_cursor.on_mouse_move)
plt.show()

捕捉資料十字游標實現

在前面的兩種實現中，滑鼠十字游標可以隨意移動。在本實現中，十字游標只會出現在離滑鼠x座標最近的資料點上。

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,line):
    self.ax = ax
    self.horizontal_line = ax.axhline(color='k',ls='--')
    self.x,self.y = line.get_data()
    self._last_index = None
    # text location in axes coords
    self.text = ax.text(0.72,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,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,ax = plt.subplots()
ax.set_title('Snapping cursor')
line,= ax.plot(x,'o')
snap_cursor = SnappingCursor(ax,line)
fig.canvas.mpl_connect('motion_notify_event',snap_cursor.on_mouse_move)
plt.show()

參考資料

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

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