Handling of Empty Panels in facet_wrap()

import pandas as pd
from lets_plot import *

LetsPlot.setup_html()

df = pd.read_csv('https://raw.githubusercontent.com/JetBrains/lets-plot-docs/master/data/mpg2.csv')
df.head(3)

	miles per gallon	number of cylinders	engine displacement (cu. inches)	engine horsepower	vehicle weight (lbs.)	time to accelerate (sec.)	model year	origin of car	vehicle name
0	18.0	8	307.0	130	3504	12.0	70	US	chevrolet chevelle malibu
1	15.0	8	350.0	165	3693	11.5	70	US	buick skylark 320
2	18.0	8	318.0	150	3436	11.0	70	US	plymouth satellite

p = (
  ggplot(df, 
         aes('engine horsepower', 'miles per gallon', 
             color=as_discrete('origin of car',order=-1)))
  + geom_point()
  + theme_grey() + ggtitle('Efficiency vs Engine Horsepower')
)
p

Empty Facet Panels are Dropped by Default

For example, there are no 3-cylinder engine cars originated in the US. Accordingly, this facet panel is dropped.

p + facet_wrap(['origin of car', 'number of cylinders'], order=[-1, 1], ncol=3)

Using the drop Parameter to Control Empty Panels

Use drop=False to show all factor levels regardless of whether they occur in the data.

p + facet_wrap(['origin of car', 'number of cylinders'], order=[-1,1], ncol=5,
               drop=False
)