Create Data Cursor with Matplotlib

Introduction

In this lab, we will learn how to create a data cursor including a text box, which shows the plot point close to the mouse pointer using Matplotlib, a plotting library for the Python programming language.

VM Tips

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Import necessary libraries

We start by importing Matplotlib and NumPy libraries. NumPy is the fundamental package for scientific computing in Python, while Matplotlib is a plotting library that produces publication-quality figures.

import matplotlib.pyplot as plt
import numpy as np

Create a plot

We create a simple plot of a parabola using NumPy's linspace function to generate 1000 values between -5 and 5 for x, and then compute y as the square of x.

fig, ax = plt.subplots(figsize=(8, 6))
ax.set_title("Cursor Tracking x Position")

x = np.linspace(-5, 5, 1000)
y = x**2

line, = ax.plot(x, y)
ax.set_xlim(-5, 5)
ax.set_ylim(0, 25)

Create the AnnotatedCursor class

We create a new class AnnotatedCursor that inherits from matplotlib.widgets.Cursor and demonstrates the creation of new widgets and their event callbacks. The AnnotatedCursor class is used to create a crosshair cursor with a text showing the current coordinates.

class AnnotatedCursor(Cursor):
    """
    A crosshair cursor like `~matplotlib.widgets.Cursor` with a text showing \
    the current coordinates.
    ...
    """

Initialize the AnnotatedCursor object

We initialize the AnnotatedCursor object by passing the plot line, line, to the AnnotatedCursor constructor.

cursor = AnnotatedCursor(line=line, ax=ax, useblit=True)

Display the plot

We display the plot using the show() method of the Matplotlib pyplot module.

plt.show()

Summary

In this lab, we learned how to create a data cursor including a text box, which shows the plot point close to the mouse pointer using Matplotlib. We created a simple plot of a parabola, then created an AnnotatedCursor object and displayed the plot.