message: fix: mutating input df in viz.py
diff: @@ -1671,6 +1671,7 @@ class DistributionBarViz(BaseViz):
  # pandas will throw away nulls when grouping/pivoting,
  # so we substitute NULL_STRING for any nulls in the necessary columns
  filled_cols = self.groupby + columns
  + df = df.copy()
  df[filled_cols] = df[filled_cols].fillna(value=NULL_STRING)
  row = df.groupby(self.groupby).sum()[metrics[0]].copy()
  