Having recently interviewed at Google and a few others, graph algorithms have been very uncommon. Trees are the most commonly discussed (directly or indirectly). In terms of algorithms, BFS and DFS are commonly used in solutions.
After BFS and DFS, THE most common topic is heaps, most often the min/max variety. You probably won't be asked to implement one, but you should understand how they work, why you would use them, and how to use one if you were given an implementation.
In my experience, if an interview question involves graphs or specifically trees, some keyword will clue you in. If there is none, formulating the problem as a graph is probably a futile exercise and you'll have to dig yourself out during the interview... Been there.
Hope that helps, best of luck!