code: def test_make_labels():
      mapping = {"y": "y", "color": ["Treatment"]}
      labels = make_labels(mapping)
      assert labels.color == "Treatment"
  
      mapping = {"y": "y", "color": ["Treatment", "Control"]}
      labels = make_labels(mapping)
      assert labels.color is None
  
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 5 for calling a global function with example usage: test_make_labels() without return types