As the (obviously biased) maintainer of both, I agree. To spell out the uses for each:

• Graph: for an object capturing "things", and "connections between things", with some well-known algorithms such as shortest-path, connected components
• PDL: for array-programming, high-performance maths including matrix maths including physics, image processing, 3D graphics, and similar data-crunching things (and anything handling binary data, really)

By the way, spoiler alert: quite a lot of graph-theory stuff can (and should) be modelled with matrix-maths stuff.