python - How to add a vertical line to a pandas bar plot of time-series data

Question

i used Pandas and supposed we have the following DataFrame :

ax = madagascar_case[["Ratio"]].loc['3/17/20':]
ax.tail()

out :

i would like to show a bar chart following ratio values and add a vertical line related to a specific date for instance : '4/20/20' :

when I try the code below :

ax = madagascar_case[["Ratio"]].loc['3/17/20':].plot.bar(figsize=(17,7), grid = True)
# to add a vertical line
ax.axvline("4/20/20",color="red",linestyle="--",lw=2 ,label="lancement")

the result is the vertical line (red) is at the wrong date and there is no label :

So to fix that I try another code by using matplotlib:

p = '4/20/20'
# Dataframe 
ax = madagascar_case[["Ratio"]].loc['3/17/20':]
# plot a histogram based on ax 
plt.hist(ax,label='ratio')
# add vertical line 
plt.axvline(p,color='g',label="lancement")

plt.legend()
plt.show()

The result was worse than expected. :

is there an easiest way to fix that ?

RVA92 >> I followed your last code :

df  = madagascar_case.loc['3/19/20':,'Ratio'].copy()
fig,ax = plt.subplots()
# plot bars 
df.plot.bar(figsize=(17,7),grid=True,ax=ax)
ax.axvline(df.index.searchsorted('4/9/20'), color="red", linestyle="--", lw=2, label="lancement")
plt.tight_layout()

the result is it works when I change the date to '4/9/20' for example , but when I change the date to '4/20/20' it doesn't fit correctly I don't know why ?

ax.axvline(df.index.searchsorted('4/20/20'), color="red", linestyle="--", lw=2, label="lancement")

Answer 1

• You can use the index number for a given date, to plot a vertical line,
  • df.index.searchsorted('3/20/20') returns the index number for the given date.

# Import libraries
import pandas as pd
import matplotlib.pyplot as plt

# Create test data
madagascar_case = pd.DataFrame(data={'Ratio': [0.5, 0.7, 0.8, 0.9]}, index=['3/19/20', '3/20/20', '3/21/20', '3/22/20'])

# Choose subset of data
df = madagascar_case.loc['3/19/20':, 'Ratio'].copy()

# Set up figure
fig, ax = plt.subplots()

# Plot bars
df.plot.bar(figsize=(17, 7), grid=True, ax=ax)

# Plot vertical lines
ax.axvline(df.index.searchsorted('3/20/20'), color="red", linestyle="--", lw=2, label="lancement")
ax.axvline(df.index.searchsorted('3/22/20'), color="red", linestyle="--", lw=2, label="lancement")