Creating a Polygon in Matplotlib

Introduction

Matplotlib is a Python library used for data visualization. In this lab, we will be using Matplotlib to create a polygon programmatically or interactively. The polygon can be used to highlight a region of interest or to mask out a certain area of a plot.

VM Tips

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Import Required Libraries

Before we start working with Matplotlib, we need to import the necessary libraries. We will be using matplotlib.pyplot and matplotlib.widgets in this lab.

import matplotlib.pyplot as plt
from matplotlib.widgets import PolygonSelector

Create a Polygon Programmatically

To create a polygon programmatically, we need to create a Figure object and an Axes object. Then, we can create a PolygonSelector object and add vertices to it. Finally, we can plot the polygon on the Axes.

fig, ax = plt.subplots()
selector = PolygonSelector(ax, lambda *args: None)

## Add three vertices
selector.verts = [(0.1, 0.4), (0.5, 0.9), (0.3, 0.2)]

## Plot the polygon
ax.add_patch(plt.Polygon(selector.verts, alpha=0.3))
plt.show()

Create a Polygon Interactively

To create a polygon interactively, we need to create a Figure object and an Axes object. Then, we can create a PolygonSelector object and add vertices to it by clicking on the plot. We can also use the shift and ctrl keys to move the vertices.

fig, ax = plt.subplots()
selector = PolygonSelector(ax, lambda *args: None)

print("Click on the figure to create a polygon.")
print("Press the 'esc' key to start a new polygon.")
print("Try holding the 'shift' key to move all of the vertices.")
print("Try holding the 'ctrl' key to move a single vertex.")

plt.show()

Putting It All Together

Let's create a complete example that includes both creating a polygon programmatically and interactively.

import matplotlib.pyplot as plt
from matplotlib.widgets import PolygonSelector

## Create a figure and axes
fig, (ax1, ax2) = plt.subplots(ncols=2, figsize=(10, 5))

## Create a PolygonSelector object and add vertices
selector1 = PolygonSelector(ax1, lambda *args: None)
selector1.verts = [(0.1, 0.4), (0.5, 0.9), (0.3, 0.2)]

## Plot the polygon
ax1.add_patch(plt.Polygon(selector1.verts, alpha=0.3))

## Create another PolygonSelector object for interactive creation
selector2 = PolygonSelector(ax2, lambda *args: None)

print("Click on the figure to create a polygon.")
print("Press the 'esc' key to start a new polygon.")
print("Try holding the 'shift' key to move all of the vertices.")
print("Try holding the 'ctrl' key to move a single vertex.")

plt.show()

Summary

In this lab, we learned how to create a polygon programmatically and interactively using Matplotlib. We also learned how to add vertices to a PolygonSelector object and plot the resulting polygon on an Axes. This knowledge can be useful for highlighting regions of interest or masking out certain areas in a plot.