Pandas Dataframe line plot display date on xaxis

Question

Compare the following code:

test = pd.DataFrame({\'date\':[\'20170527\',\'20170526\',\'20170525\'],\'ratio1\':[1,0.98,0.97]})
test[\'date\'] = pd.to_datetime         


        

Answer 1

In general the datetime utilities of pandas and matplotlib are incompatible. So trying to use a matplotlib.dates object on a date axis created with pandas will in most cases fail.

One reason is e.g. seen from the documentation

datetime objects are converted to floating point numbers which represent time in days since 0001-01-01 UTC, plus 1. For example, 0001-01-01, 06:00 is 1.25, not 0.25.

However, this is not the only difference and it is thus advisable not to mix pandas and matplotlib when it comes to datetime objects.

There is however the option to tell pandas not to use its own datetime format. In that case using the matplotlib.dates tickers is possible. This can be steered via.

df.plot(x_compat=True)

Since pandas does not provide sophisticated formatting capabilities for dates, one can use matplotlib for plotting and formatting.

import pandas as pd
import matplotlib.pyplot as plt
import matplotlib.dates as dates

df = pd.DataFrame({'date':['20170527','20170526','20170525'],'ratio1':[1,0.98,0.97]})
df['date'] = pd.to_datetime(df['date'])

usePandas=True
#Either use pandas
if usePandas:
    df = df.set_index('date')
    df.plot(x_compat=True)
    plt.gca().xaxis.set_major_locator(dates.DayLocator())
    plt.gca().xaxis.set_major_formatter(dates.DateFormatter('%d\n\n%a'))
    plt.gca().invert_xaxis()
    plt.gcf().autofmt_xdate(rotation=0, ha="center")
# or use matplotlib
else:
    plt.plot(df["date"], df["ratio1"])
    plt.gca().xaxis.set_major_locator(dates.DayLocator())
    plt.gca().xaxis.set_major_formatter(dates.DateFormatter('%d\n\n%a'))
    plt.gca().invert_xaxis()

plt.show()