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 pltimport numpy as npclass 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 pltimport numpy as npclass 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 pltimport numpy as npclass 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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