code: def test_stat_function(self):
      p = (
          ggplot(self.df1, aes(x="var"))
          + geom_point(aes(y="var"))
          + stat_function(fun=lambda x: x)
          + scale_y_log10()
      )
      assert p == "test_stat_function"
  
file_path: plotnine/tests/test_aes.py
imports: import numpy
  import pandas
  from plotnine import aes, after_stat, geom_col, geom_crossbar, geom_point, ggplot, scale_x_log10, scale_y_log10, stage, stat_bin_2d, stat_ecdf, stat_function
  from plotnine.mapping.aes import make_labels
label_desc: Use image node_id 1 for calling the TestTransScale obj's underlying member method code with example usage: obj.test_stat_function() without return types