Warnings when using Pandas 1.4

[drhagen]

In Pandas 1.4.0, the .append method of data frames and series was deprecated. This causes warnings to appear in Jupyter when using lots of plotnine functions which use .append. For example, the fourth example from the README:

from plotnine import *
from plotnine.data import mtcars

(ggplot(mtcars, aes('wt', 'mpg', color='factor(gear)'))
 + geom_point()
 + stat_smooth(method='lm')
 + facet_wrap('~gear'))

[has2k1]

Resolved in e9cc70d.