How to Annotate Matplotlib Scatter Plots?

A scatter plot uses dots to represent values for two different numeric variables. In Python, we have a library matplotlib in which there is a function called scatter that helps us to create Scatter Plots. Here, we will use matplotlib.pyplot.scatter() method to plot.

Syntax : matplotlib.pyplot.scatter(x,y)

Parameters:

• x and y are float values and are the necessary parameters to create a scatter plot
• marker : MarkerStyle, default: rcParams[“scatter.marker”] (default: ‘o’)
• cmap : cmapstr or Colormap, default: rcParams[“image.cmap”] (default: ‘viridis’)
• linewidths : float or array-like, default: rcParams[“lines.linewidth”] (default: 1.5)
• alpha : float, default: None → represents the transparency

Annotation of matplotlib means that we want to place a piece of text next to the scatter. There can be two cases depending on the number of the points we have to annotate :

1. Single point annotation
2. All points annotation

Single Point annotation

In single-point annotation we can use matplotlib.pyplot.text and mention the x coordinate of the scatter point and y coordinate + some factor so that text can be distinctly visible from the plot, and then we have to mention the text.

Syntax: matplotlib.pyplot.text( x, y, s)

Parameters:

• x, y : scalars — The position to place the text. By default, this is in data coordinates. The coordinate system can be changed using the transform parameter.
• s : str — The text.
• fontsize — It is an optional parameter used to set the size of the font to be displayed.

Approach:

1. Import libraries.
2. Create data.
3. Make scatter plot.
4. Apply plt.text() method.

Implementation:

Python3

# Importing libraries
import matplotlib.pyplot as plt

# Preparing dataset
x = [x for x in range(10)]
y = [5, 4, 4, 8, 5, 6, 8, 7, 1, 3]

# plotting scatter plot
plt.scatter(x, y)

# annotation of the third point
plt.text(2,4.2,"third")
plt.show()

Output:

All points annotation

If we want to annotate all points in the scatter plot then matplotlib.pyplot has an inbuilt function annotate which takes the text, x, and y coordinates of the point.

Syntax: matplotlib.pyplot.annotate( text, xy )

Parameters:

• text : str — The text of the annotation. s is a deprecated synonym for this parameter.
• xy : (float, float) — The point (x, y) to annotate. The coordinate system is determined by xy coordinates.

Approach:

1. Import libraries.
2. Create data.
3. Store all the annotations in a list in order with the sequence of the points to be displayed.
4. Draw the scatter plot.
5. Using a for loop annotate each point.

Implementation:

Python3

# Importing libraries
import matplotlib.pyplot as plt

# Preparing dataset
x = [x for x in range(10)]
y = [5, 2, 4, 8, 5, 6, 8, 7, 1, 3]
text = ["first", "second", "third", "fourth", "fifth",
        "sixth", "seventh", "eighth", "ninth", "tenth"]

# plotting scatter plot
plt.scatter(x, y)

# Loop for annotation of all points
for i in range(len(x)):
    plt.annotate(text[i], (x[i], y[i] + 0.2))

# adjusting the scale of the axes
plt.xlim((-1, 10))
plt.ylim((0, 10))
plt.show()

Output: