fixing graph division

As the title says.
For some reason the graph division was using 
```
Graph.errorsy()
```
instead of 
```
Graph.yerr()
```
to get the lower and upper errors.

I hope this fix is OK (it does not work without).

I was also thinking about adding a test for it, but it seems that the test folder is almost empty. Didn't you use to have a quite good test coverage? 

Cheers,
Luke