Pick events for data interaction in Matplotlib - Step-by-Step Execution

Concept Flow - Pick events for data interaction

Plot data points

↓

User clicks near a point

↓

Pick event triggered

↓

Identify picked point

↓

Perform action (highlight, print info)

↓

Wait for next pick event or exit

The flow shows how a user click near a plotted point triggers a pick event, which identifies the point and performs an action like highlighting or printing info.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
points = ax.plot([1,2,3], [4,5,6], 'o', picker=5)
def onpick(event):
    print(f"Picked point: {event.artist.get_xdata()[event.ind[0]]}, {event.artist.get_ydata()[event.ind[0]]}")
fig.canvas.mpl_connect('pick_event', onpick)
plt.show()

This code plots points and listens for pick events; when a point is clicked, it prints the coordinates.

Execution Table

Step	Action	Event Triggered	Picked Point Index	Output
1	Plot points at (1,4), (2,5), (3,6)	No	N/A	Points shown on plot
2	User clicks near (2,5)	Yes	1	Picked point: 2, 5
3	User clicks near (3,6)	Yes	2	Picked point: 3, 6
4	User clicks away from points	No	N/A	No output
5	User closes plot window	No	N/A	Program ends

💡 User closes plot window, ending the event loop

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
event.ind	N/A	[1]	[2]	N/A
event.artist.get_xdata()	[1,2,3]	[1,2,3]	[1,2,3]	N/A
event.artist.get_ydata()	[4,5,6]	[4,5,6]	[4,5,6]	N/A

Key Moments - 2 Insights

Why does clicking away from points not trigger a pick event?

Because the picker tolerance is set to 5 pixels, clicks outside this range do not trigger the pick event, as shown in execution_table step 4.

How does the program know which point was picked?

The pick event provides an index list 'event.ind' that identifies which point(s) were clicked, as seen in steps 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the picked point index when the user clicks near (2,5)?

A 0

B 2

C 1

D None

💡 Hint

Check the 'Picked Point Index' column at step 2 in execution_table.

At which step does the program output 'Picked point: 3, 6'?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Output' column in execution_table for the message with coordinates (3,6).

If the picker tolerance was set to 0, what would happen when clicking near points?

A Pick events would trigger on any click

B Pick events would trigger only on exact clicks on points

C No pick events would trigger

D All points would be picked simultaneously

💡 Hint

Picker tolerance controls how close the click must be to a point to trigger the event.

Concept Snapshot

Pick events let users click near plotted points to interact.
Set picker tolerance to define click sensitivity.
Use 'pick_event' to catch clicks on points.
Event gives index of picked points.
Use this to highlight or show info about points.

Full Transcript

This visual execution shows how matplotlib pick events work. First, points are plotted with a picker tolerance. When the user clicks near a point, a pick event triggers. The event provides the index of the picked point. The program then prints the coordinates of that point. Clicking away from points does not trigger the event. The event loop continues until the user closes the plot window.

Practice

1. What does setting the picker parameter on a plot element in matplotlib do?

easy

A. Removes the plot element from the figure

B. Makes the plot element respond to mouse clicks for interaction

C. Saves the plot element as an image file

D. Changes the color of the plot element

Solution

1. Step 1: Understand the role of the picker parameter

   The picker parameter enables a plot element to detect mouse clicks or pick events.

2. Step 2: Connect picker to interaction

   When picker is set, the element becomes clickable, allowing interaction like showing data details.

3. Final Answer:

   Makes the plot element respond to mouse clicks for interaction -> Option B

4. Quick Check:

   picker enables click interaction = D [OK]

Hint: picker makes plot elements clickable for interaction [OK]

Common Mistakes:

• Confusing picker with color or style changes
• Thinking picker saves images
• Assuming picker removes elements

2. Which of the following is the correct way to connect a pick event handler function named on_pick to a matplotlib figure fig?

easy

A. fig.mpl_connect('pick_event', on_pick)

B. fig.connect('pick_event', on_pick)

C. fig.canvas.mpl_connect('pick_event', on_pick)

D. fig.canvas.connect('pick_event', on_pick)

Solution

1. Step 1: Recall the correct method to connect events in matplotlib

   Events are connected using mpl_connect on the figure's canvas object.

2. Step 2: Match the syntax for pick events

   The correct syntax is fig.canvas.mpl_connect('pick_event', handler_function).

3. Final Answer:

   fig.canvas.mpl_connect('pick_event', on_pick) -> Option C

4. Quick Check:

   Use fig.canvas.mpl_connect for events = A [OK]

Hint: Use fig.canvas.mpl_connect to link pick events [OK]

Common Mistakes:

• Using fig.connect instead of fig.canvas.mpl_connect
• Calling mpl_connect on fig instead of fig.canvas
• Using connect instead of mpl_connect

3. Consider the code below:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
line, = ax.plot([1, 2, 3], [4, 5, 6], picker=5)
def on_pick(event):
    print(f"Picked point: {event.ind}")
fig.canvas.mpl_connect('pick_event', on_pick)
plt.show()

What will happen when you click near the second point on the line?

medium

A. The program prints 'Picked point: [1]' indicating the second point was picked

B. Nothing happens because picker=5 is invalid

C. An error occurs because on_pick is not connected properly

D. The plot closes immediately

Solution

1. Step 1: Understand picker=5 meaning

   Setting picker=5 means clicks within 5 points of the line points trigger pick events.

2. Step 2: Analyze on_pick behavior on clicking second point

   Clicking near the second point triggers on_pick, printing the index of that point, which is 1 (zero-based).

3. Final Answer:

   The program prints 'Picked point: [1]' indicating the second point was picked -> Option A

4. Quick Check:

   picker=5 triggers pick near points = C [OK]

Hint: picker=5 allows clicks near points to trigger events [OK]

Common Mistakes:

• Thinking picker=5 is invalid
• Assuming event.ind is not available
• Believing on_pick is not connected

4. The following code is intended to print the index of a picked point on a scatter plot, but it raises an error:

import matplotlib.pyplot as plt
fig, ax = plt.subplots()
sc = ax.scatter([1,2,3], [4,5,6], picker=True)
def on_pick(event):
    print(event.ind)
fig.mpl_connect('pick_event', on_pick)
plt.show()

What is the main error causing the failure?

medium

A. Calling mpl_connect on fig instead of fig.canvas

B. Using picker=True instead of a numeric tolerance

C. Not defining on_pick before connecting it

D. Using scatter instead of plot for pick events

Solution

1. Step 1: Check how event connection is done

   The code calls fig.mpl_connect, but the correct method is fig.canvas.mpl_connect.

2. Step 2: Understand impact of wrong connection

   Because mpl_connect is not a method of fig, this causes an AttributeError and failure.

3. Final Answer:

   Calling mpl_connect on fig instead of fig.canvas -> Option A

4. Quick Check:

   Use fig.canvas.mpl_connect, not fig.mpl_connect = A [OK]

Hint: Always connect events on fig.canvas, not fig [OK]

Common Mistakes:

• Using picker=True is allowed, not an error
• Assuming on_pick must be defined before connection
• Thinking scatter can't use pick events

5. You want to create an interactive matplotlib scatter plot where clicking a point highlights it by changing its color. Which approach correctly combines pick events and updating the plot?

hard

A. Set picker on scatter points, connect pick_event to a function that prints point coordinates only

B. Set picker on the figure, not on points, and change colors in the handler

C. Use plt.show() inside the pick event handler to refresh the plot

D. Set picker on scatter points, connect pick_event to a function that changes the point's color and calls fig.canvas.draw()

Solution

1. Step 1: Enable picking on scatter points

   Set the picker parameter on scatter plot points to detect clicks on them.

2. Step 2: Update point color and redraw figure in handler

   In the pick event handler, change the color of the selected point and call fig.canvas.draw() to update the display.

3. Final Answer:

   Set picker on scatter points, connect pick_event to a function that changes the point's color and calls fig.canvas.draw() -> Option D

4. Quick Check:

   picker + color change + canvas.draw() = B [OK]

Hint: Change color in handler and redraw with fig.canvas.draw() [OK]

Common Mistakes:

• Only printing coordinates without updating plot
• Calling plt.show() inside event handler causes errors
• Setting picker on figure instead of points