matplotlib页面上有一个示例，您可以对其进行调整，以在感兴趣的位置显示一个点。

import matplotlib.pyplot as plt
import matplotlib.widgets as widgets
import numpy as np


class SnaptoCursor(object):
    def __init__(self, ax, x, y):
        self.ax = ax
        self.ly = ax.axvline(color='k', alpha=0.2)  # the vert line
        self.marker, = ax.plot([0],[0], marker="o", color="crimson", zorder=3) 
        self.x = x
        self.y = y
        self.txt = ax.text(0.7, 0.9, '')

    def mouse_move(self, event):
        if not event.inaxes: return
        x, y = event.xdata, event.ydata
        indx = np.searchsorted(self.x, [x])[0]
        x = self.x[indx]
        y = self.y[indx]
        self.ly.set_xdata(x)
        self.marker.set_data([x],[y])
        self.txt.set_text('x=%1.2f, y=%1.2f' % (x, y))
        self.txt.set_position((x,y))
        self.ax.figure.canvas.draw_idle()

t = np.arange(0.0, 1.0, 0.01)
s = np.sin(2*2*np.pi*t)
fig, ax = plt.subplots()

#cursor = Cursor(ax)
cursor = SnaptoCursor(ax, t, s)
cid =  plt.connect('motion_notify_event', cursor.mouse_move)

ax.plot(t, s,)
plt.axis([0, 1, -1, 1])
plt.show()

请看这里：Is there a matplotlib equivalent of MATLAB's datacursormode?

import matplotlib.pyplot as plt

class DataCursor(object):
    text_template = 'x: %0.2f\ny: %0.2f'
    x, y = 0.0, 0.0
    xoffset, yoffset = -20, 20
    text_template = 'x: %0.2f\ny: %0.2f'

    def __init__(self, ax):
        self.ax = ax
        self.annotation = ax.annotate(self.text_template, 
                xy=(self.x, self.y), xytext=(self.xoffset, self.yoffset), 
                textcoords='offset points', ha='right', va='bottom',
                bbox=dict(boxstyle='round,pad=0.5', fc='yellow', alpha=0.5),
                arrowprops=dict(arrowstyle='->', connectionstyle='arc3,rad=0')
                )
        self.annotation.set_visible(False)

    def __call__(self, event):
        self.event = event
        # xdata, ydata = event.artist.get_data()
        # self.x, self.y = xdata[event.ind], ydata[event.ind]
        self.x, self.y = event.mouseevent.xdata, event.mouseevent.ydata
        if self.x is not None:
            self.annotation.xy = self.x, self.y
            self.annotation.set_text(self.text_template % (self.x, self.y))
            self.annotation.set_visible(True)
            event.canvas.draw()

fig = plt.figure()
line, = plt.plot(range(10), 'ro-')
fig.canvas.mpl_connect('pick_event', DataCursor(plt.gca()))
line.set_picker(5) # Tolerance in points
plt.show()