You can use the following syntax to add a legend to a scatterplot in Matplotlib:

import matplotlib.pyplotas plt
from matplotlib.colorsimport ListedColormap

#define values, classes, and colors to map 
values = [0, 0, 1, 2, 2, 2]
classes = ['A', 'B', 'C']
colors = ListedColormap(['red', 'blue', 'purple'])

#create scatterplot
scatter = plt.scatter(x, y, c=values, cmap=colors)

#add legend
plt.legend(*scatter.legend_elements())

The following examples show how to use this syntax in practice.

Example 1: Scatterplot Legend with Values

The following example shows how to create a scatterplot where the legend displays values:

import matplotlib.pyplotas plt
from matplotlib.colorsimport ListedColormap

#define data
x = [3, 4, 4, 6, 8, 9]
y = [12, 14, 17, 16, 11, 13]

#define values, classes, and colors to map 
values = [0, 0, 1, 2, 2, 2]
classes = ['A', 'B', 'C']
colors = ListedColormap(['red', 'blue', 'purple'])

#create scatterplot
scatter = plt.scatter(x, y, c=values, cmap=colors)

#add legend with values
plt.legend(*scatter.legend_elements())

Example 2: Scatterplot Legend with Classes

The following example shows how to create a scatterplot where the legend displays class names:

import matplotlib.pyplotas plt
from matplotlib.colorsimport ListedColormap

#define data
x = [3, 4, 4, 6, 8, 9]
y = [12, 14, 17, 16, 11, 13]

#define values, classes, and colors to map 
values = [0, 0, 1, 2, 2, 2]
classes = ['A', 'B', 'C']
colors = ListedColormap(['red', 'blue', 'purple'])

#create scatterplot
scatter = plt.scatter(x, y, c=values, cmap=colors)

#add legend with class names
plt.legend(handles=scatter.legend_elements()[0], labels=classes)

Notice that this legend displays the class names that we specified (A, B, C) as opposed to the values (0, 1, 2) that we specified.