Fix bug of network graph(#2847) and trivial code clean up.

### Before

The lines of develop parents are overlap.

![fix_overlap_before](https://f.cloud.github.com/assets/72138/113938/7519e8ce-6b83-11e2-995f-e7e90fa93b0c.png)

### After

The lines of develop parents are not overlap.

![fix_overlap_after](https://f.cloud.github.com/assets/72138/113940/7dd06fb0-6b83-11e2-9a7c-a652736154f2.png)