5 Best Ways to Make Hollow Square Marks with Matplotlib in Python

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πŸ’‘ Problem Formulation: When visualizing data with scatter plots in Matplotlib, a common requirement is to mark points with shapes that are not filled, such as hollow squares. These markers can improve the readability and aesthetics of plots, especially when overlaying multiple datasets. This article explores how to create scatter plots with hollow square markers using Python’s Matplotlib library, ensuring your data points stand out clearly.

Method 1: Utilizing Marker Styles

In Matplotlib, the scatter() function allows for a variety of marker styles. To make hollow square marks, you specify the marker parameter to ‘s’ for square and set facecolors to ‘none’. This method is straightforward and leverages built-in Matplotlib styles.

Here’s an example:

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y = [10, 20, 25, 30]
plt.scatter(x, y, edgecolors='blue', facecolors='none', marker='s')
plt.show()

Output: A scatter plot with blue hollow square markers at the provided (x, y) coordinates.

This code snippet creates a simple scatter plot with hollow square markers by setting edgecolors to ‘blue’ and facecolors to ‘none’. The marker style ‘s’ is used for square markers, and calling plt.show() displays the plot.

Method 2: Custom Path Creation

For more control over the appearance, you can create a custom marker using Matplotlib’s Path module. This approach requires constructing a path that defines the outline of a hollow square, which can then be passed to the scatter() function via the marker parameter.

Here’s an example:

import matplotlib.pyplot as plt
from matplotlib.path import Path
import matplotlib.patches as patches

square_vertices = [(1, -1), (1, 1), (-1, 1), (-1, -1), (0, 0)]
square_codes = [Path.MOVETO, Path.LINETO, Path.LINETO, Path.LINETO, Path.CLOSEPOLY]
square_path = Path(square_vertices, square_codes)

x = [1, 2, 3, 4]
y = [10, 20, 25, 30]
plt.scatter(x, y, edgecolor='green', facecolor='none', marker=square_path)
plt.show()

Output: A scatter plot with green hollow square markers that have been custom-defined.

The code snippet defines a square using a series of vertices and path codes to move between them. A Path object is created and used as the marker shape in the scatter() function. The edgecolor is set to green, while the facecolor remains ‘none’ to keep the square hollow.

Method 3: Using Unicode Characters

Another option is to use Unicode characters that resemble a hollow square as markers. Matplotlib can use specific Unicode characters as scatter plot markers by specifying the marker parameter with the appropriate Unicode.

Here’s an example:

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y = [10, 20, 25, 30]
plt.scatter(x, y, color='red', marker=u'$\u25A1$', s=100)  # Unicode for hollow square
plt.show()

Output: A scatter plot with red hollow square markers represented by a Unicode character.

In this snippet, the square Unicode symbol '\u25A1' is used as the marker in the scatter() function. The color parameter is used to set the color to red, and s controls the size of the marker. The plot is then displayed using plt.show().

Method 4: Overlaying Markers

Creating hollow squares can also be achieved by overlaying a smaller filled square on top of a larger square. This creates the illusion of a hollow marker. You need to plot two sets of markers for each point, adjusting the sizes to create a border-like effect.

Here’s an example:

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y = [10, 20, 25, 30]
base_size = 100  # Size of the larger, base square

# Plot larger, base squares
plt.scatter(x, y, s=base_size, marker='s', color='blue')
# Plot smaller, overlay squares with the background color, creating the "hollow" effect
plt.scatter(x, y, s=base_size / 2, marker='s', color='white')
plt.show()

Output: A scatter plot with what appears to be hollow square markers created by overlaying two different sized squares.

This code creates a layered effect by first plotting larger blue squares and then overlaying them with smaller white squares. By setting the base_size variable, we can control the size of the markers easily, creating a hollow effect.

Bonus One-Liner Method 5: Using Scatter Plot with Edgecolor and Facecolor

As a quick one-liner solution, combine the scatter() function with explicit settings for edgecolor and facecolor, forming a hollow square marker with minimal code.

Here’s an example:

import matplotlib.pyplot as plt

plt.scatter([1, 2, 3], [3, 2, 1], marker='s', edgecolor='black', facecolor='none')
plt.show()

Output: A minimalistic scatter plot with black hollow square markers.

This one-liner effectively plots hollow square markers by combining the built-in ‘s’ marker with an edgecolor and leaving the facecolor as ‘none’ to indicate no fill. It’s a concise way to achieve the desired marker style.

Summary/Discussion

  • Method 1: Utilizing Marker Styles. Simple and easy to use. Limited customization options.
  • Method 2: Custom Path Creation. Highly customizable. Requires more code and understanding of Path semantics.
  • Method 3: Using Unicode Characters. Easy and quick. Dependent on available Unicode characters and may not scale well with the plot size.
  • Method 4: Overlaying Markers. Creates a unique hollow effect. May be less efficient due to plotting markers twice.
  • Method 5: Bonus One-Liner. Extremely concise. Lacks customization and can be limited in marker size adjustments.