Support for constructing Charts with only y-values

This simplifies constructing simple plots without supplying
explicit x-values. This more closely matches the matplotlib
API making it easier for new users to get started and
addresses issue #133.

holoviews/element/chart.py

@@ -45,10 +45,13 @@ def __init__(self, data, **params):
 
 
     def _validate_data(self, data):
-        if data.ndim > 1 and not data.shape[1] == len(self.dimensions()):
+        if data.ndim == 1:
+            data = np.array(zip(range(len(data)), data))
+        if not data.shape[1] == len(self.dimensions()):
             raise ValueError("Data has to match number of key and value dimensions")
         return data
 
+
     def _process_map(self, ndmap):
         """
         Base class to process an NdMapping to be collapsed into a Chart.