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.