fix single ax bug

Author: jiyuang

tools/plot-tools/abacus_plot/dos.py

@@ -391,7 +391,7 @@ def _parial_plot(self,
         Returns:
             DOSPlot object: for manually plotting picture with dosplot.ax 
         """
-        if not isinstance(ax, list):
+        if isinstance(ax, axes.Axes):
             ax = [ax]
 
         dos, totnum = parse_projected_data(self.orbitals, species, keyname)