Cursor and event handling in Matplotlib - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the module needed for plotting.

Matplotlib

import [1] as plt

Drag options to blanks, or click blank then click option'

Amatplotlib.pyplot

Bnumpy

Cseaborn

Dpandas

Attempts:

3 left

💡 Hint

Common Mistakes

Importing numpy or pandas instead of matplotlib.pyplot.

Using seaborn which is a different plotting library.

We use matplotlib.pyplot for plotting graphs and handling cursor events.

2fill in blank

medium

Complete the code to create a figure and axis for plotting.

Matplotlib

fig, ax = plt.[1]()

Drag options to blanks, or click blank then click option'

Aplot

Bsubplots

Cfigure

Dshow

Attempts:

3 left

💡 Hint

Common Mistakes

Using plt.figure() which returns only the figure.

Using plt.plot() which creates a plot but not figure and axes separately.

The subplots() function creates a figure and axes for plotting.

3fill in blank

hard

Fix the error in the event connection code to correctly connect the cursor move event.

Matplotlib

cid = fig.canvas.[1]('motion_notify_event', on_move)

Drag options to blanks, or click blank then click option'

Aadd_event

Bconnect_event

Cconnect

Dbind

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect method names like connect_event or add_event.

Trying to use bind which is not a matplotlib method.

The correct method to connect events in matplotlib is connect.

4fill in blank

hard

Fill both blanks to create a cursor object and enable it on the axis.

Matplotlib

cursor = Cursor([1], useblit=True, color='red', linewidth=1)
cursor.[2]()

Drag options to blanks, or click blank then click option'

Aax

Bfig

Cdraw

Denable

Attempts:

3 left

💡 Hint

Common Mistakes

Passing figure instead of axis to Cursor.

Using a non-existent method like enable() instead of draw().

The cursor is created on the axis ax and draw() is called to enable it.

5fill in blank

hard

Fill all three blanks to create a function that prints cursor coordinates on mouse move.

Matplotlib

def on_move(event):
    if event.inaxes == [1]:
        x, y = event.[2], event.[3]
        print(f"Cursor at: ({x:.2f}, {y:.2f})")

Drag options to blanks, or click blank then click option'

Aax

Bxdata

Cydata

Dfig

Attempts:

3 left

💡 Hint

Common Mistakes

Checking event.inaxes against figure instead of axis.

Using event.x and event.y which are pixel positions, not data coordinates.

The event's inaxes is compared to ax. Coordinates are accessed via xdata and ydata.

Practice

1. What is the main purpose of using mpl_connect in matplotlib?

easy

A. To create a new figure window

B. To connect an event like mouse click to a custom function

C. To save the current plot as an image file

D. To change the color of the plot lines

Solution

1. Step 1: Understand what mpl_connect does

   mpl_connect links events such as mouse clicks or key presses to functions you define.

2. Step 2: Identify the correct purpose

   It does not save images, create figures, or change colors directly. It connects events to functions.

3. Final Answer:

   To connect an event like mouse click to a custom function -> Option B

4. Quick Check:

   Event connection = C [OK]

Hint: Remember: mpl_connect links events to your functions [OK]

Common Mistakes:

• Thinking mpl_connect saves or modifies plots directly
• Confusing mpl_connect with figure creation
• Assuming mpl_connect changes plot styles

2. Which of the following is the correct syntax to connect a mouse click event to a function named on_click using matplotlib?

easy

A. fig.connect_event('button_press', on_click)

B. fig.mpl_connect('click', on_click)

C. fig.canvas.mpl_connect('button_press_event', on_click)

D. fig.connect('mouse_click', on_click)

Solution

1. Step 1: Recall the correct event name for mouse clicks

   The correct event name in matplotlib for mouse button press is 'button_press_event'.

2. Step 2: Check the syntax of mpl_connect

   The method is called on the figure's canvas as fig.canvas.mpl_connect(event_name, function).

3. Final Answer:

   fig.canvas.mpl_connect('button_press_event', on_click) -> Option C

4. Quick Check:

   Correct event name and syntax = A [OK]

Hint: Use 'button_press_event' for mouse clicks with mpl_connect [OK]

Common Mistakes:

• Using wrong event names like 'click' or 'mouse_click'
• Using non-existent methods like connect_event
• Mixing up method names and event strings

3. Consider the code below:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def on_move(event):
    print(f"Mouse at: {event.xdata}, {event.ydata}")

cid = fig.canvas.mpl_connect('motion_notify_event', on_move)
plt.show()

What will happen when you move the mouse over the plot area?

medium

A. The coordinates of the mouse pointer inside the plot will be printed continuously

B. Nothing will happen because the event is not connected properly

C. The plot will close immediately

D. An error will occur because event.xdata is undefined

Solution

1. Step 1: Understand the event type 'motion_notify_event'

   This event triggers whenever the mouse moves over the figure canvas.

2. Step 2: Analyze the function on_move

   The function prints the mouse coordinates inside the plot area using event.xdata and event.ydata.

3. Final Answer:

   The coordinates of the mouse pointer inside the plot will be printed continuously -> Option A

4. Quick Check:

   Mouse move event prints coords = B [OK]

Hint: motion_notify_event tracks mouse movement over plot [OK]

Common Mistakes:

• Assuming no output because event is not connected
• Thinking event.xdata is always None or undefined
• Expecting plot to close on mouse move

4. The following code is intended to print the mouse button pressed, but it raises an error:

import matplotlib.pyplot as plt

fig, ax = plt.subplots()

def on_click(event):
    print(f"Button pressed: {event.button}")

fig.canvas.mpl_connect('button_press_event', on_click())
plt.show()

What is the error and how to fix it?

medium

A. The function is called immediately; remove parentheses in mpl_connect

B. The event name is wrong; use 'mouse_press' instead

C. event.button does not exist; use event.key instead

D. mpl_connect should be called on ax, not fig

Solution

1. Step 1: Identify the error in function connection

   Using on_click() calls the function immediately instead of passing it as a reference.

2. Step 2: Correct the function reference in mpl_connect

   Remove parentheses to pass the function itself: fig.canvas.mpl_connect('button_press_event', on_click).

3. Final Answer:

   The function is called immediately; remove parentheses in mpl_connect -> Option A

4. Quick Check:

   Pass function, not call it = D [OK]

Hint: Pass function name without () to mpl_connect [OK]

Common Mistakes:

• Calling the function instead of passing it
• Using wrong event names
• Trying to access wrong event attributes

5. You want to create an interactive plot where clicking inside the plot area prints the nearest data point's coordinates from a scatter plot. Which approach correctly combines cursor event handling and data lookup?

hard

A. Use plt.scatter() with a parameter to automatically print nearest point on click

B. Use 'motion_notify_event' to print coordinates continuously without checking points

C. Connect 'key_press_event' to print data points when any key is pressed

D. Connect 'button_press_event' to a function that calculates distances from click to all points and prints nearest

Solution

1. Step 1: Identify the correct event for mouse clicks

   Use 'button_press_event' to detect mouse clicks inside the plot.

2. Step 2: Implement logic to find nearest data point

   Calculate distances from click position to all scatter points, then print the closest one.

3. Step 3: Verify other options

   'motion_notify_event' prints continuously, 'key_press_event' is unrelated, and plt.scatter has no auto-print feature.

4. Final Answer:

   Connect 'button_press_event' to a function that calculates distances from click to all points and prints nearest -> Option D

5. Quick Check:

   Click event + nearest point logic = A [OK]

Hint: Use click event plus distance check to find nearest point [OK]

Common Mistakes:

• Using motion event instead of click for selection
• Expecting built-in auto-print in scatter
• Confusing key press with mouse click events