Question: The drop parameter of facet_grid seems not work

[bioinformatist]

Hi, it seems drop parameter of facet_grid() function do not work.
Below is official example:

import pandas as pd

from plotnine import *
from plotnine.data import *

%matplotlib inline

(
    ggplot(mpg, aes(x='displ', y='hwy'))
    + geom_point()
    + facet_grid('drv ~ .')
    + labs(x='displacement', y='horsepower')
)

Then I can add a new category for mpg dataset as:

mpg['drv'] = pd.Categorical(mpg['drv'], ['4', 'f', 'r', 'yingyingying'])

This time the column mpg['drv'] seems like this:

An empty category 'yingyingying' was added.

I add drop=False to my code, but nothing happens:

(
    ggplot(mpg, aes(x='displ', y='hwy'))
    + geom_point()
    + facet_grid('drv ~ .', drop=False)
    + labs(x='displacement', y='horsepower')
)

How can I add an empty panel of 'yingyingying' without any data points?

Please tell me if there's any misunderstanding.

Thank you in advance.