create a 1D histogram from a 1D graph

Full circle:
```python
>>> from rootpy.plotting import Hist, Graph
>>> hist = Hist(10, 0, 1)
>>> hist.FillRandom('gaus')
Hist('740ad6ac7ccb469f9218315eeca59200')
>>> graph = Graph(hist)
>>> hist2 = Hist(graph)
```