code: def test_facet_grid_expression():
      p = g + facet_grid(
          ["var2", "pd.cut(var1, (0, 2, 4), include_lowest=True)"]
      )
      assert p == "facet_grid_expression"
  
imports: import numpy
  import pandas
  from plotnine import aes, annotate, facet_grid, facet_wrap, geom_abline, geom_path, geom_point, ggplot
  from plotnine.data import mpg, mtcars
label_desc: Use image node_id 9 for calling a global function with example usage: test_facet_grid_expression() without return types