Mplcursors for hover labels in Matplotlib - Step-by-Step Execution

Concept Flow - Mplcursors for hover labels

Create plot with points

↓

Attach mplcursors to plot

↓

Hover over a point?

No→Wait

Yes↓

Show label with point info

↓

Move cursor away?

No→Keep label

Yes↓

Hide label

↓

End

The flow shows creating a plot, attaching mplcursors, then showing or hiding labels when hovering over points.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
import mplcursors

fig, ax = plt.subplots()
points = ax.scatter([1,2,3], [4,5,6])
cursor = mplcursors.cursor(points)
plt.show()

This code plots three points and attaches mplcursors to show labels when hovering.

Execution Table

Step	Action	Evaluation	Result
1	Create figure and axes	fig, ax = plt.subplots()	Figure and axes objects created
2	Plot points	ax.scatter([1,2,3], [4,5,6])	Scatter plot with 3 points created
3	Attach mplcursors	cursor = mplcursors.cursor(points)	Cursor object attached to points
4	Hover over point at (1,4)?	Yes	Label with coordinates (1, 4) shown
5	Move cursor away	Yes	Label hidden
6	Hover over point at (2,5)?	Yes	Label with coordinates (2, 5) shown
7	Move cursor away	Yes	Label hidden
8	Hover over point at (3,6)?	Yes	Label with coordinates (3, 6) shown
9	Move cursor away	Yes	Label hidden
10	No more hover	No	Wait for user action

💡 User stops hovering, program waits or ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	After Step 9
fig	None	Figure object	Figure object	Figure object	Figure object	Figure object	Figure object	Figure object	Figure object
ax	None	Axes object	Axes object	Axes object	Axes object	Axes object	Axes object	Axes object	Axes object
points	None	Scatter object with 3 points	Scatter object with 3 points	Scatter object with 3 points	Scatter object with 3 points	Scatter object with 3 points	Scatter object with 3 points	Scatter object with 3 points	Scatter object with 3 points
cursor	None	None	Cursor object attached	Cursor object attached	Cursor object attached	Cursor object attached	Cursor object attached	Cursor object attached	Cursor object attached
label_visible	False	False	False	True (point 1)	False	True (point 2)	False	True (point 3)	False

Key Moments - 2 Insights

Why does the label only appear when I hover exactly over a point?

The mplcursors cursor is attached to the scatter points, so it only triggers labels when the mouse is close enough to a point, as shown in steps 4, 6, and 8 in the execution_table.

What happens if I move the cursor away from the point?

The label disappears as shown in steps 5, 7, and 9 in the execution_table, because mplcursors hides the label when the cursor is no longer hovering over the point.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What is the label_visible state?

A False

B True (point 1)

C True (point 2)

D None

💡 Hint

Check the variable_tracker row for label_visible at After Step 4.

At which step does the label for point (2,5) appear?

A Step 6

B Step 4

C Step 8

D Step 2

💡 Hint

Look at execution_table rows where label shows coordinates.

If you remove mplcursors.cursor(points), what changes in the execution?

A Labels still appear on hover

B Plot does not show points

C Labels never appear on hover

D Program crashes

💡 Hint

mplcursors.cursor attaches the hover label functionality as shown in step 3.

Concept Snapshot

Mplcursors adds interactive hover labels to matplotlib plots.
Use mplcursors.cursor(scatter) to attach.
Labels show when hovering near points.
Labels hide when cursor moves away.
No labels appear without mplcursors.

Full Transcript

This visual execution shows how mplcursors works with matplotlib scatter plots. First, a figure and axes are created. Then points are plotted with scatter. Next, mplcursors.cursor is attached to the points. When the user moves the mouse over a point, a label with the point's coordinates appears. Moving the mouse away hides the label. This repeats for each point hovered. Variables like fig, ax, points, cursor, and label visibility change step by step. Key moments clarify why labels only appear on hover and disappear when moving away. The quiz tests understanding of label visibility states and the role of mplcursors.

Practice

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

easy

A. To save the plot as an image file

B. To change the color of the plot lines

C. To add interactive hover labels showing data values

D. To create 3D plots automatically

Solution

1. Step 1: Understand mplcursors functionality

   mplcursors is a tool that adds interactive hover labels to matplotlib plots, showing data values when you hover over points.

2. Step 2: Compare options with mplcursors purpose

   Changing colors, saving images, or creating 3D plots are not related to hover labels, so only adding interactive hover labels fits.

3. Final Answer:

   To add interactive hover labels showing data values -> Option C

4. Quick Check:

   mplcursors = interactive hover labels [OK]

Hint: Remember mplcursors = hover labels for data points [OK]

Common Mistakes:

• Confusing mplcursors with plot styling tools
• Thinking mplcursors saves files
• Assuming mplcursors creates 3D plots

2. Which of the following is the correct way to import mplcursors for use in a matplotlib plot?

easy

A. import mplcursors

B. from matplotlib import mplcursors

C. import matplotlib.mplcursors

D. import mpl_cursor

Solution

1. Step 1: Recall correct import syntax

   The mplcursors library is a separate package and is imported simply with import mplcursors.

2. Step 2: Check other options for errors

   from matplotlib import mplcursors is wrong because mplcursors is not part of matplotlib. import matplotlib.mplcursors is wrong because it's a separate package, not a submodule. import mpl_cursor is incorrect because the module name is mplcursors, not mpl_cursor.

3. Final Answer:

   import mplcursors -> Option A

4. Quick Check:

   Correct import = import mplcursors [OK]

Hint: Use simple import: import mplcursors [OK]

Common Mistakes:

• Trying to import mplcursors from matplotlib
• Thinking mplcursors is matplotlib.mplcursors submodule
• Using wrong module name like mpl_cursor

3. What will be the output behavior of this code snippet?

import matplotlib.pyplot as plt
import mplcursors

fig, ax = plt.subplots()
points = ax.plot([1, 2, 3], [4, 5, 6], 'o')
mplcursors.cursor(points)
plt.show()

medium

A. Plot shows points with hover labels displaying (x, y) values

B. Plot shows points but no hover labels appear

C. Code raises an error because cursor() needs extra arguments

D. Plot shows a line connecting points without markers

Solution

1. Step 1: Understand code components

   The code plots points at (1,4), (2,5), (3,6) with markers 'o'. Then mplcursors.cursor(points) adds interactive hover labels.

2. Step 2: Predict output behavior

   When running plt.show(), the plot appears with points. Hovering over points shows labels with their coordinates because mplcursors is activated correctly.

3. Final Answer:

   Plot shows points with hover labels displaying (x, y) values -> Option A

4. Quick Check:

   mplcursors.cursor(points) = hover labels shown [OK]

Hint: mplcursors.cursor() adds hover labels to plotted points [OK]

Common Mistakes:

• Thinking cursor() needs extra arguments
• Expecting no hover labels without extra setup
• Confusing line plot with marker plot

4. Identify the error in this code that prevents hover labels from showing:

import matplotlib.pyplot as plt
import mplcursors

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

medium

A. The variable 'line' should be a list, not a single Line2D object

B. The plot command is missing marker style to show points

C. mplcursors is not imported correctly

D. mplcursors.cursor() must be called before plotting

Solution

1. Step 1: Analyze plot and cursor usage

   The code plots a line without markers. Hover labels appear on points, but here points are not visible because no markers are set.

2. Step 2: Identify why hover labels don't show

   mplcursors works on plotted points. Without markers, the line is continuous and no discrete points exist to hover on, so labels don't appear.

3. Final Answer:

   The plot command is missing marker style to show points -> Option B

4. Quick Check:

   Missing markers = no hover labels [OK]

Hint: Add markers to plot for mplcursors hover labels [OK]

Common Mistakes:

• Thinking mplcursors must be called before plot
• Assuming single Line2D object is invalid input
• Believing import error causes no labels

5. You want to show hover labels only for points where y > 5 in this plot. Which code change achieves this?

import matplotlib.pyplot as plt
import mplcursors

fig, ax = plt.subplots()
x = [1, 2, 3, 4]
y = [4, 5, 6, 7]
points = ax.plot(x, y, 'o')
# Add hover labels only for y > 5
mplcursors.cursor(points)

hard

A. Use mplcursors.cursor(points).remove() for points with y <= 5

B. Filter points before plotting: ax.plot([xi for xi, yi in zip(x,y) if yi>5], [yi for yi in y if yi>5], 'o')

C. Set cursor with mplcursors.cursor(points, hover=True, filter=lambda sel: sel.target[1] > 5)

D. Use mplcursors.cursor(points).connect('add', lambda sel: sel.annotation.set_visible(sel.target[1] > 5))

Solution

1. Step 1: Understand how to filter hover labels

   mplcursors allows connecting to events like 'add' to customize annotation visibility based on data values.

2. Step 2: Analyze options for filtering by y > 5

   Use mplcursors.cursor(points).connect('add', lambda sel: sel.annotation.set_visible(sel.target[1] > 5)) uses a lambda to set annotation visible only if y > 5, which is correct. Filter points before plotting: ax.plot([xi for xi, yi in zip(x,y) if yi>5], [yi for yi in y if yi>5], 'o') filters points before plotting but does not use mplcursors filtering. Set cursor with mplcursors.cursor(points, hover=True, filter=lambda sel: sel.target[1] > 5) uses a non-existent 'filter' argument. Use mplcursors.cursor(points).remove() for points with y <= 5 tries to remove cursor which is not valid for selective filtering.

3. Final Answer:

   Use mplcursors.cursor(points).connect('add', lambda sel: sel.annotation.set_visible(sel.target[1] > 5)) -> Option D

4. Quick Check:

   Connect 'add' event to filter hover labels [OK]

Hint: Use cursor.connect('add', lambda sel: condition) to filter labels [OK]

Common Mistakes:

• Trying to filter points only by plotting
• Using unsupported 'filter' argument in cursor()
• Attempting to remove cursor for selective points