Mplcursors for hover labels in Matplotlib

Introduction

Mplcursors lets you add labels that appear when you hover over points on a plot. This helps you see details without cluttering the graph.

You want to show extra info about points only when the mouse is over them.

You have a scatter plot and want to display values interactively.

You want to explore data points in a plot without adding static text.

You need a simple way to add interactive labels in matplotlib plots.

Syntax

Matplotlib

import mplcursors

cursor = mplcursors.cursor(plot_elements)
cursor.connect("add", lambda sel: sel.annotation.set_text('your label text'))

mplcursors.cursor() activates hover labels on given plot elements like lines or scatter points.

The connect method lets you customize the label text when hovering.

Examples

This example shows hover labels on scatter points displaying their coordinates.

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)
cursor.connect("add", lambda sel: sel.annotation.set_text(f"Point: {sel.target}"))

plt.show()

Hover labels on a line plot show the index and y-value of each point.

Matplotlib

import matplotlib.pyplot as plt
import mplcursors

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

cursor = mplcursors.cursor(line)
cursor.connect("add", lambda sel: sel.annotation.set_text(f"Index: {sel.index}\nValue: {sel.target[1]}"))

plt.show()

Sample Program

This program creates a scatter plot and shows x and y values when you hover over each point.

Matplotlib

import matplotlib.pyplot as plt
import mplcursors

# Create a scatter plot
fig, ax = plt.subplots()
x = [10, 20, 30, 40]
y = [1, 4, 9, 16]
scatter = ax.scatter(x, y)

# Add hover labels showing x and y values
cursor = mplcursors.cursor(scatter)
cursor.connect("add", lambda sel: sel.annotation.set_text(f"x={sel.target[0]}\ny={sel.target[1]}"))

plt.title('Scatter plot with hover labels')
plt.xlabel('X axis')
plt.ylabel('Y axis')
plt.show()

Important Notes

Make sure you have mplcursors installed: run pip install mplcursors.

Hover labels only appear when you move the mouse over the plot points.

You can customize the label text fully using the connect method.

Summary

Mplcursors adds interactive hover labels to matplotlib plots.

Use it to show details about points without cluttering the graph.

Customize labels easily with a simple function.

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