code: def test_dodge_preserve_single():
      data1 = pd.DataFrame({"x": ["a", "b", "b"], "y": ["a", "a", "b"]})
      p = ggplot(data1, aes("x", fill="y")) + geom_bar(
          position=position_dodge(preserve="single")
      )
      assert p == "dodge_preserve_single"
  
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 8 for calling a global function with example usage: test_dodge_preserve_single() without return types