Add Text to Graph Combine

I am combining three graphs using 'graph combine'. By default they appear in a 2 x 2 arrangement with the lower right slot empty. I'd like to add text to this empty area. What's the best way to do this?

I have tried the 'caption' and 'note' options, with and without the 'position' suboption, but that distorts the shape or is at the far bottom of the combined graph.

I don't know if this would work, but if I could either save the text as a standalone .gph file, I could add that way, or perhaps there is an option I'm missing for 'graph combine' whereby you can just place text anywhere you like (using coordinates, not clock position).