Respect series order in stacked area plot (#5236)

This is a quick fix of #5235, until a more thorough refactoring (if ever) as discussed there.

holoviews/element/chart.py

@@ -281,7 +281,7 @@ def stack(cls, areas, baseline_name='Baseline'):
         vdims = [vdim, baseline_name]
         baseline = None
         stacked = areas.clone(shared_data=False)
-        for key, sdf in df.groupby(level=levels):
+        for key, sdf in df.groupby(level=levels, sort=False):
             sdf = sdf.droplevel(levels).reindex(index=df.index.unique(-1), fill_value=0)
             if baseline is None:
                 sdf[baseline_name] = 0