Avoid dtype=object in group plotting

Author: mroeschke

pandas/plotting/_matplotlib/boxplot.py

@@ -400,7 +400,7 @@ def plot_group(keys, values, ax: Axes, **kwds):
             ax.set_ylabel(pprint_thing(ylabel))
 
         keys = [pprint_thing(x) for x in keys]
-        values = [np.asarray(remove_na_arraylike(v), dtype=object) for v in values]
+        values = [remove_na_arraylike(v) for v in values]
         bp = ax.boxplot(values, **kwds)
         if fontsize is not None:
             ax.tick_params(axis="both", labelsize=fontsize)