add comment on constants

edvart/report_sections/bivariate_analysis.py

@@ -984,6 +984,7 @@ def contingency_table(
     )
     if size_factor == "auto":
         n_digits_max = np.floor(np.log10(table.max().max()))
+        # Constants chosen empirically to make the numbers fit in the cells
         size_factor = max(
             0.72,
             0.18 * (1 + n_digits_max),