Fix drag-n-drop plotting to respect distribution flag.

Fixes trac issue [#10743](http://trac.mantidproject.org/mantid/ticket/10743)

Tester: The following script (reproduced from the ticket)
```python
w=Load('CNCS_7860')
w=SumSpectra(w)
w=Rebin(w,10)
```
produces a workspace. Plot a spectrum from the workspace and then once the plot is open drag the same workspace on to the plot. The curves should now match.