Matplotlib – How to show the coordinates of a point upon mouse click?

In Matplotlib, you can capture mouse click coordinates on a plot by connecting an event handler to the figure's canvas. This is useful for interactive data exploration and annotation.

Setting Up Mouse Click Detection

The key is to use mpl_connect() to bind a function to the 'button_press_event' ?

import numpy as np
import matplotlib.pyplot as plt

plt.rcParams["figure.figsize"] = [7.00, 3.50]
plt.rcParams["figure.autolayout"] = True

def mouse_event(event):
    if event.xdata is not None and event.ydata is not None:
        print('x: {:.3f} and y: {:.3f}'.format(event.xdata, event.ydata))

fig = plt.figure()
cid = fig.canvas.mpl_connect('button_press_event', mouse_event)

x = np.linspace(-10, 10, 100)
y = np.sin(x)

plt.plot(x, y, 'b-', linewidth=2)
plt.title('Click on the plot to see coordinates')
plt.xlabel('X values')
plt.ylabel('Y values')
plt.grid(True, alpha=0.3)

plt.show()

When you click on different points of the plot, the coordinates will be printed to the console ?

x: -3.099 and y: -0.014
x: -0.287 and y: -0.207
x: -3.028 and y: -0.184
x: -5.770 and y: 0.424
x: -3.918 and y: 0.684

Enhanced Version with Visual Markers

You can also add visual markers at clicked points and display coordinates on the plot ?

import numpy as np
import matplotlib.pyplot as plt

class ClickPlotter:
    def __init__(self):
        self.fig, self.ax = plt.subplots(figsize=(8, 5))
        self.clicked_points = []
        
    def mouse_event(self, event):
        if event.xdata is not None and event.ydata is not None:
            x, y = event.xdata, event.ydata
            print('Clicked at x: {:.3f}, y: {:.3f}'.format(x, y))
            
            # Add marker at clicked point
            self.ax.plot(x, y, 'ro', markersize=8)
            
            # Add text annotation
            self.ax.annotate('({:.2f}, {:.2f})'.format(x, y), 
                           (x, y), xytext=(5, 5), 
                           textcoords='offset points',
                           fontsize=9, color='red')
            
            self.fig.canvas.draw()
            self.clicked_points.append((x, y))

# Create plotter instance
plotter = ClickPlotter()

# Connect the event
cid = plotter.fig.canvas.mpl_connect('button_press_event', plotter.mouse_event)

# Create sample data
x = np.linspace(-5, 5, 100)
y = x**2 - 2*x + 1

plotter.ax.plot(x, y, 'b-', linewidth=2, label='y = x² - 2x + 1')
plotter.ax.set_title('Click to Mark Points and Show Coordinates')
plotter.ax.set_xlabel('X values')
plotter.ax.set_ylabel('Y values')
plotter.ax.grid(True, alpha=0.3)
plotter.ax.legend()

plt.show()

How It Works

The event handler receives an event object containing:

• event.xdata: X-coordinate in data units
• event.ydata: Y-coordinate in data units
• event.button: Mouse button pressed (1=left, 2=middle, 3=right)
• event.inaxes: The axes where the click occurred

Key Points

• Always check if event.xdata and event.ydata are not None
• Use fig.canvas.draw() to refresh the plot after adding markers
• Store the connection ID if you need to disconnect the event later
• Multiple event handlers can be connected to the same figure

Conclusion

Mouse click coordinate detection in Matplotlib enables interactive data exploration. Use mpl_connect() with 'button_press_event' to capture clicks and access coordinates through event.xdata and event.ydata.