ggnet -- simple network plots with ggplot2

`ggnet` is a function to plot simple networks with ggplot2:

![](https://github.com/briatte/ggnet/raw/master/example1.png)

See the [full-fledged example](https://github.com/briatte/ggnet) using French MPs on Twitter.

__Update (July 5):__ after a few weeks of testing, the function now has more arguments and more examples. The node colours are now robust to missing values. I have also [fixed](https://github.com/briatte/ggnet/issues/1) the namespace conflicts that could occur due to similarly-named functions in the `ergm`, `igraph`, `network` and `sna` packages.