Additional “axis like” plot in matplotlib pandas plot

Question

I am storing some data in pandas dataframe. Also, I am using matplotlib to create plots showing the data. Please look at this pretty picture:

Red line shows some values corresponding to x axis points. It is just a column in the dataframe. I want to add additional annotation categorizing x axis points. Those categories are stored as additional columns in the original dataframe. It does not have to look exactly like in the picture.

The goal is to somehow show x axis ranges categorization. What is a smart and elegant way to add such an annotation?

Answer 1

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

# Create sample data
df = pd.DataFrame({'Data': [np.sin(i) + 3 for i in np.arange(1, 11, 0.1)],
                   'Annotation': ['A'] * 10 + ['B'] * 20 + [np.nan] * 10 +
                                 ['C'] * 10 + ['D'] * 10 + [np.nan] * 20 +
                                 ['D'] * 20})
# Get unique annotations
annotation_symbols = [i for i in df['Annotation'].unique() if not pd.isnull(i)]
# Transform each unique text annotation into a new column,
# where ones represent the corresponding annotation being 'active' and
# NaNs represent the corresponding annotation being 'inactive'
df = pd.concat([df, pd.get_dummies(df['Annotation']).replace(0, np.nan)])

plt.style.use('ggplot')  # Let's use nicer style
ax = plt.figure(figsize=(7, 5)).add_subplot(111)
df.plot.line(x=df.index, y='Data', ax=ax)
df.plot.line(x=df.index, y=annotation_symbols, ax=ax)

Produced figure: