GraphView: fix width of the vertical labels

The same width value was used for both horizontal and vertical labels,
making the vertical labels too wide when the horizontal labels are
wider than the real width of the vertical labels. This fix makes use
of the real width of the vertical labels.

Before and after the fix:

![before](https://f.cloud.github.com/assets/342101/955122/ae187cfa-0424-11e3-8c36-103e53cc443d.png) ![after](https://f.cloud.github.com/assets/342101/955123/b34f7bc4-0424-11e3-9e86-7ad699d52c52.png)



.