Added general __len__ implementation for Element

holoviews/core/element.py

@@ -187,6 +187,15 @@ def array(self, dimensions=[]):
         return np.column_stack(columns)
 
 
+    def __len__(self):
+        """
+        Elements must define a length, by default will simply be
+        computed from dimension values, subclasses may override this
+        for efficiency and correctness.
+        """
+        return len(self.dimension_values(0))
+
+
 
 class Tabular(Element):
     """