How Does Marker Size (Parameter `s`) Work in Matplotlib Scatter Plots?

Understanding Marker Size in Pyplot Scatter Plots

In creating scatter plots using Pyplot, the documentation specifies a parameter s that determines the size of the markers. This parameter takes values in a unit called "points^2." This can be puzzling, leaving you wondering what exactly this unit represents.

What is a "Point^2"?

A "point^2" is a measurement of area. It does not directly correspond to the pixel dimensions of the marker. Instead, it specifies the area that the marker occupies, regardless of its shape. For example, a circular marker with radius r has an area of πr^2. If you set s to 100, it means that the area of the marker is 100 "points^2."

Scaling the Marker Size

Understanding the unit "points^2" is crucial for scaling the marker size appropriately. To double the width or height of a circular marker, you need to increase s by a factor of 4. This is because the area of a circle increases quadratically with its radius.

Visualizing the Marker Size

To illustrate this concept, consider the following two scatter plots:

• In the first plot, s is set to 20*4^n, where n varies from 0 to 5. This exponential increase in area results in a dramatic increase in the apparent marker size.
• In the second plot, s is set to 20*2^n. This quadratic increase in area results in a more intuitive increase in the apparent marker size.

Customizing the Marker Size

The exact value of a "point" is arbitrary and can be adjusted to suit your needs. By scaling the values of s by a constant, you can achieve the desired marker size without worrying about the exact meaning of the "points^2" unit.

Additional Notes:

Note that the above discussion pertains to circular markers. For other shapes, the relationship between s and the apparent marker size can differ.

To visualize this relationship for different size functions, consider the following scatter plot:

This plot shows how exponential, square, and linear size functions affect the apparent size of the markers.

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