code: def test_df_hexbin_C():
      df = pd.DataFrame(
          {"x": range(10), "y": range(10), "C": range(10)}
      )
      gridsize = 10
      plot = df.vgplot.hexbin(x="x", y="y", C="C", gridsize=gridsize)
      assert plot.mark == "rect"
      utils.check_encodings(plot, x="x", y="y", color="C")
      assert plot["encoding"]["x"]["bin"] == alt.Bin(maxbins=gridsize)
      assert plot["encoding"]["y"]["bin"] == alt.Bin(maxbins=gridsize)
      assert plot["encoding"]["color"]["aggregate"] == "mean"
  
imports: import pytest
  import pandas
  import altair
  from pdvega.tests import utils
label_desc: Use image node_id 25 for calling a global function with example usage: test_df_hexbin_C() without return types