Handle by argument for gridded types (#461)

* Handle by argument for gridded types

* Fixed bug

hvplot/converter.py

@@ -934,6 +934,8 @@ def __call__(self, kind, x, y):
         zs = self.kwds.get('z', [])
         if not isinstance(zs, list): zs = [zs]
         groups += self.grid
+        if self.gridded and not kind == 'points':
+            groups += self.by
         if groups or len(zs) > 1:
             if self.streaming:
                 raise NotImplementedError("Streaming and groupby not yet implemented")
@@ -983,6 +985,9 @@ def method_wrapper(ds, x, y):
                                   kdims=dimensions)
             else:
                 obj = method(x, y, data=dataset.data)
+
+            if self.gridded and self.by and not kind == 'points':
+                obj = obj.layout(self.by) if self.subplots else obj.overlay(self.by) 
             if self.grid:
                 obj = obj.grid(self.grid).opts(shared_xaxis=True, shared_yaxis=True)
         else: