facet_wrap: indescriptive error when the specified facet variable is not present in the dataset.

[alshan]

from lets_plot import *
import pandas as pd
LetsPlot.setup_html()

mpg = pd.read_csv('https://raw.githubusercontent.com/JetBrains/lets-plot-docs/master/data/mpg2.csv')

ggplot(mpg, 
    aes('engine horsepower', 'miles per gallon', 
        color='origin of car')) + geom_point() + facet_wrap('number of cylinders')

OK

but if there is a wrong name in facet_wrap(), then error:

axis length: NaN