Fix custom benchmark plotting for selected portfolio columns

tests/test_plotting.py

@@ -1220,6 +1220,12 @@ def test_cum_returns_benchmark_trace(self):
         assert "Value" in nt
         assert isinstance(nt["Value"], go.Scatter)
         assert len(nt["Value"].y) == len(index_5)
+        # Custom benchmark returns should also work when plotting a selected portfolio column.
+        fig = pf_multi.plot_cum_returns(column="a", benchmark_rets=pf_multi["b"].returns())
+        nt = named_traces(fig)
+        assert "Benchmark" in nt
+        assert isinstance(nt["Benchmark"], go.Scatter)
+        assert len(nt["Benchmark"].y) == len(index_5)
 
     def test_smoke_plot_drawdowns(self):
         self._assert_fig(

vectorbt/portfolio/base.py

@@ -5385,7 +5385,7 @@ def plot_cum_returns(
         if benchmark_rets is None:
             benchmark_rets = self.benchmark_returns(group_by=group_by)
         else:
-            benchmark_rets = broadcast_to(benchmark_rets, self.obj)
+            benchmark_rets = broadcast_to(benchmark_rets, self.wrapper.dummy(group_by=group_by))
         benchmark_rets = self.select_one_from_obj(benchmark_rets, self.wrapper.regroup(group_by), column=column)
         kwargs = merge_dicts(
             dict(