Use index in pandas to plot data

You can use reset_index to turn the index back into a column:

monthly_mean.reset_index().plot(x='index', y='A')

Look at monthly_mean.reset_index() by itself- the date is no longer in the index, but is a column in the dataframe, which is now just indexed by integers. If you look at the documentation for reset_index, you can get a bit more control over the process, including assigning sensible names to the index.

Try this,

monthly_mean.plot(y='A', use_index=True)

monthly_mean.plot(y='A')

Uses index as x-axis by default.