I'm not sure see what use the humungous single graph at full resolution would be. Hardly anyone on Earth would even be able to open it for decades.

You might consider starting by generating a very large number of reasonably small subgraphs as individual graphics. If you want to give the impression of a giant map that can be zoomed, you might start like this: take each the image of each subgraph and make a smaller, reduced resolution image of it. Group a number of subgraphs together and tile them, using a convenient tile size, on the order of 100 pixels square or so; then make a reduced resolution image out of that. Lather, rinse, and repeat until you finally get one set of tiles that are collectively small enough to display in a single window, say around 800x600 pixels or so.

Yes, it is likely to be little more than a blob at that scale.

Now, on the display side, that's a bit like how map programs such as Google Earth or Yahoo Maps work as you zoom in starting from a view of the entire planet, and they had a very similar problem to solve. You may or may not need or want the fancy Ajax scrolling. For many uses, the old way of clicking on an image map to zoom in at the clicked location is good enough. Either way, I think the underlying idea of tiles that are all the same pixel size but have different scales as you zoom in and out may be the simplest way of handling a graph that is really much too big to handle all at once.