code: def test_df_hexbin_Cfunc():
      df = pd.DataFrame(
          {"x": range(10), "y": range(10), "C": range(10)}
      )
      plot = df.vgplot.hexbin(
          x="x", y="y", C="C", reduce_C_function=min
      )
      assert plot["encoding"]["color"]["aggregate"] == "min"
      utils.check_encodings(plot, x="x", y="y", color="C")
  
imports: import pytest
  import pandas
  import altair
  from pdvega.tests import utils
label_desc: Use image node_id 26 for calling a global function with example usage: test_df_hexbin_Cfunc() without return types