Matplotlib: Annotations and Drawing on Subplots

Annotations and Drawing on Subplots

•  Once the subplots are created, we can annotate the text or draw horizontal or vertical lines on each subplot.

1) Annotation text

• We use the annotate() function for writing the text on the subplot.

Syntax

plt.annotate(text, xy, xytext, arrowprops)

Where

text: The annotation text

xy: Coordinates of the point to annotate (x, y)

xytext: Position of the annotation text

arrowprops: Style of arrow connecting text and point

2) Drawing lines and shapes

•  We can draw simple shapes or highlight regions using following functions.

plt.axhline(y,color,linestyle)  : ‒    Draws horizontal line

plt.axvline(x,color,linestyle)  :‒   Draws vertical line

plt.axhspan(ymin,ymax,color,alpha)  :‒   Fills horizontal region

plt.axvspan(xmin,xmax,color,alpha) :‒   Fills vertical region

Demo example

•  Following is a simple Python programs in which we are annotating and drawing the lines on  the subplots.

Python program

import matplotlib.pyplot as plt

fig, axes = plt.subplots(1,2, figsize=(8,3))

x = [1,2,3,4,5]

y1 = [2,4,6,8,10]

y2 = [1,3,2,5,4]

axes[0].plot(x,y1)

axes[0].set_title("Plot#1")

axes[0].axhline(y=sum(y1)/len(y1), linestyle="‒‒") # average line

axes[0].annotate("Peak", xy=(5,10), xytext=(4,9), arrowprops= dict(arrowstyle="‒>"))

axes[1].plot(x,y2)

axes[1].set_title("Plot#2")

axes[1].axvspan(3,5, alpha=0.12)                 # highlight x range

axes[1].text(3.2, 2.5, "busy period")

fig.suptitle("Subplots with Annotations & Drawings")

plt.tight_layout()

plt.show()

Output

Code explanation: In above code,

1) We have created two subplots with titles Plot#1 and Plot#2.

For Plot 1

2) axes[0].axhline(y=sum(y1)/len(y1), linestyle‒'‒‒'): This draws the horizontal dashed line at the average value of y1.

3) axes[0].annotate("Peak", xy=(5,10), xytext=(4,9), arrowprops‒dict(arrowstyle ="‒>")) : This adds an annotation labeled "Peak" pointing to the highest point (5, 10) with an arrow from (4, 9).

For Plot 2

4)axes[1].axvspan(3, 5, alpha=0.12): Highlights the x‒range from 3 to 5 with a translucent vertical band.

5) axes[1].text(3.2, 2.5, "busy period"): Adds a text label "busy period" inside the highlighted region.

For entire plot

6) fig.suptitle("Subplots with Annotations & Drawings"): Adds the super title for the entire plot.

7) plt.tight_layout(): Adjusts spacing between subplots to prevent overlap.

Finally the complete plot is displayed.