[Tut] How to Color a Scatter Plot by Category using Matplotlib in Python

[Tut] How to Color a Scatter Plot by Category using Matplotlib in Python - Printable Version

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[Tut] How to Color a Scatter Plot by Category using Matplotlib in Python - xSicKxBot - 04-15-2022

How to Color a Scatter Plot by Category using Matplotlib in Python

Problem Formulation

Given three arrays:

The first two arrays
Code:
x
and
Code:
y
of length
Code:
n
contain the
Code:
(x_i, y_i)
data of a 2D coordinate system.
The third array
Code:
c
provides categorical label information so we essentially get
Code:
n
data bundles
Code:
(x_i, y_i, c_i)
for an arbitrary number of categories
Code:
c_i
.

Question: How to plot the data so that

Code:
(x_i, y_i)

and

Code:
(x_j, y_j)

with the same category

Code:
c_i == c_j

have the same color?

Solution: Use Pandas groupby() and Call plt.plot() Separately for Each Group

To plot data by category, you iterate over all groups separately by using the

Code:
data.groupby()

operation. For each group, you execute the

Code:
plt.plot()

operation to plot only the data in the group.

In particular, you perform the following steps:

Use the
Code:
data.groupby("Category")
function assuming that data is a Pandas DataFrame containing the
Code:
x
,
Code:
y
, and
Code:
category
columns for n data points (rows).
Iterate over all
Code:
(name, group)
tuples in the grouping operation result obtained from step one.
Use
Code:
plt.plot(group["X"], group["Y"], marker="o", linestyle="", label=name)
to plot each group separately using the
Code:
x
,
Code:
y
data and
Code:
name
as a label.

Here’s what that looks like in code:

Code:
import pandas as pd
import matplotlib.pyplot as plt # Generate the categorical data
x = [1, 2, 3, 4, 5, 6]
y = [42, 41, 40, 39, 38, 37]
c = ['a', 'b', 'a', 'b', 'b', 'a'] data = pd.DataFrame({"X": x, "Y": y, "Category": c})
print(data) # Plot data by category
groups = data.groupby("Category")
for name, group in groups: plt.plot(group["X"], group["Y"], marker="o", linestyle="", label=name) plt.legend()
plt.show()

Before I show you how the resulting plot looks, allow me to show you the data output from the

Code:
print()

function. Here’s the output of the categorical data:

Code:
X Y Category
1 42 a
2 41 b
3 40 a
4 39 b
5 38 b
6 37 a

Now, how does the colored category plot look like? Here’s how:

[Image: image-61.png]

If you want to learn more about Matplotlib, feel free to check out our full blog tutorial series: