Cursor and event handling in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this matplotlib cursor event code?

Consider the following code that uses matplotlib to track mouse clicks on a plot. What will be printed when you click inside the plot area?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()
ax.plot([1, 2, 3], [4, 5, 6])

class ClickTracker:
    def __init__(self, ax):
        self.ax = ax
        self.cid = ax.figure.canvas.mpl_connect('button_press_event', self.onclick)

    def onclick(self, event):
        if event.inaxes == self.ax:
            print(f'Clicked at x={event.xdata:.2f}, y={event.ydata:.2f}')

tracker = ClickTracker(ax)
plt.show()

A Prints the coordinates of the click inside the plot area with two decimals.

B Prints the pixel coordinates of the click on the screen.

C Raises an AttributeError because event.xdata does not exist.

D Prints nothing because the event handler is not connected.

💡 Hint

Check what event.xdata and event.ydata represent in matplotlib mouse events.

The button_press_event provides event.xdata and event.ydata which are the data coordinates of the click inside the axes. The handler prints these coordinates formatted to two decimals.

❓ data_output

intermediate

How many times is the event handler called when clicking outside the axes?

Given this matplotlib event handler connected to button_press_event, how many times will the onclick method print output if you click 3 times outside the plot axes?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

class ClickCounter:
    def __init__(self, ax):
        self.ax = ax
        self.count = 0
        self.cid = ax.figure.canvas.mpl_connect('button_press_event', self.onclick)

    def onclick(self, event):
        if event.inaxes == self.ax:
            self.count += 1
            print(f'Click count inside axes: {self.count}')

counter = ClickCounter(ax)
plt.show()

A 0 times, because clicks outside axes do not trigger the print.

B 3 times, one for each click regardless of location.

C Raises an error because event.inaxes is None outside axes.

D 1 time, because only the first click outside axes triggers the event.

💡 Hint

Look at the condition if event.inaxes == self.ax and what event.inaxes is when clicking outside.

When clicking outside the axes, event.inaxes is None, so the condition fails and the print statement is never executed.

❓ visualization

advanced

Which option correctly updates a plot title with cursor position on mouse move?

Which code snippet below will update the plot's title to show the current mouse position inside the axes as you move the cursor?

A

def on_move(event):
    if event.inaxes:
        event.inaxes.set_title(f'x={event.xdata}, y={event.ydata}')
        event.canvas.draw_idle()

fig.canvas.mpl_connect('motion_notify_event', on_move)

B

def on_move(event):
    if event.inaxes:
        event.inaxes.set_title(f'x={event.xdata:.1f}, y={event.ydata:.1f}')
        event.canvas.draw_idle()

fig.canvas.mpl_connect('motion_notify_event', on_move)

C

def on_move(event):
    if event.inaxes:
        event.inaxes.title.set_text(f'x={event.x}, y={event.y}')
        event.canvas.draw()

fig.canvas.mpl_connect('button_press_event', on_move)

D

def on_move(event):
    if event.inaxes:
        event.inaxes.set_title(f'x={event.xdata:.1f}, y={event.ydata:.1f}')
        event.canvas.flush_events()

fig.canvas.mpl_connect('motion_notify_event', on_move)

💡 Hint

Check the event type for mouse movement and how to update the title with formatted floats.

Option B uses the correct event motion_notify_event, formats coordinates to one decimal, updates the title, and calls draw_idle() for efficient redraw.

Option B uses wrong event and wrong coordinate attributes. Option B misses formatting decimals. Option B uses flush_events() which is less appropriate here.

🔧 Debug

advanced

Why does this matplotlib event handler not print anything on click?

Examine the code below. Why does clicking on the plot not produce any printed output?

Matplotlib

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

class Handler:
    def __init__(self, ax):
        self.ax = ax
        self.cid = ax.figure.canvas.mpl_connect('button_press_event', self.on_click)

    def on_click(self, event):
        if event.inaxes == self.ax:
            print('Clicked!')

handler = Handler(ax)
plt.show()

A The event handler is connected correctly, but the plot window is not shown.

B The condition compares event.inaxes to ax, but event.inaxes is None when clicking inside.

C The event handler is connected to the figure canvas, but the method name is misspelled.

D The code works correctly and prints 'Clicked!' when clicking inside the axes.

💡 Hint

Check if the event handler is connected and if the condition matches clicks inside the axes.

The event handler is connected properly to button_press_event. The condition event.inaxes == self.ax is true when clicking inside the axes. The plot window is shown with plt.show(). So the code prints 'Clicked!' as expected.

🚀 Application

expert

How to implement a draggable vertical line cursor in matplotlib?

You want to add a vertical line cursor to a matplotlib plot that the user can drag horizontally with the mouse. Which approach below correctly implements this behavior?

A Use <code>axvline</code> to draw the line, connect <code>motion_notify_event</code> only, and update the line's x-position without checking mouse button state.

B Use <code>axhline</code> to draw the line, connect only <code>button_press_event</code> to update the line's y-position, and call <code>plt.show()</code> inside the event handler.

C Use <code>axvline</code> to draw the line, connect <code>button_press_event</code>, <code>motion_notify_event</code>, and <code>button_release_event</code> to update the line's x-position while dragging, and call <code>canvas.draw_idle()</code> after updates.

D Use <code>plot</code> to draw a vertical line, connect <code>key_press_event</code> to move the line horizontally with arrow keys, and call <code>canvas.flush_events()</code> after updates.

💡 Hint

Think about which events are needed to track dragging and how to update the line position smoothly.

Option C correctly uses axvline to draw the vertical line and connects mouse press, move, and release events to track dragging. It updates the line's x-position and calls canvas.draw_idle() for efficient redraw.

Option C uses horizontal line and only press event, so no dragging. Option C uses keyboard events, not mouse dragging. Option C misses mouse button state check, so dragging won't work properly.