Warn when aesthetics are dropped during stat processing

Author: has2k1

plotnine/stats/stat.py

@@ -2,6 +2,7 @@
 
 import typing
 from copy import deepcopy
+from warnings import warn
 
 import pandas as pd
 
@@ -33,6 +34,13 @@
     "na_rm": False,
 }
 
+DROPPED_TPL = """
+The following aesthetics were dropped during processing: {dropped}.
+plotnine could not infer the correct grouping.
+Did you forget to specify a `group` aesthetic or to convert a numerical \
+variable into a categorial?
+"""
+
 
 class stat(ABC, metaclass=Register):
     """Base class of all stats"""
@@ -307,6 +315,9 @@ def compute_panel(self, data: pd.DataFrame, scales: pos_scales):
             stats.append(group_result)
 
         stats = pd.concat(stats, axis=0, ignore_index=True)
+        dropped = data.columns.difference(stats.columns).to_list()
+        if dropped:
+            warn(DROPPED_TPL.format(dropped=dropped))
         # Note: If the data coming in has columns with non-unique
         # values with-in group(s), this implementation loses the
         # columns. Individual stats may want to do some preparation