code: def test_stack():
      p = ggplot(data2, aes("factor(z)")) + geom_bar(
          aes(fill="factor(x)"), position="stack"
      )
      assert p == "stack"
  
imports: import string
  import numpy
  import pandas
  import pytest
  from plotnine import aes, after_stat, geom_bar, geom_boxplot, geom_col, geom_jitter, geom_point, geom_rect, geom_segment, geom_text, ggplot, position_dodge, position_dodge2, position_jitter, position_jitterdodge, position_nudge, position_stack, scale_y_log10, stage
  from plotnine.exceptions import PlotnineError
  from plotnine.positions.position import position
label_desc: Use image node_id 3 for calling a global function with example usage: test_stack() without return types