As a slight aside, I'm attempting to do this as it will save on IO, and I wont have to write a scavange routine to trash the old graphs.

As a counter-point to the desire to write a scavange routine to clean up old graph image files, I would direct your attention to aspects of temporary file handling described previously by myself here - In particular I would direct your attention to the File::Temp module which provides methods for creating and working with temporary files as anonymous STDIO streams which are not stored beyond the life of the file handle.

This approach, while still encompassing the aspect of IO hits, would allow you to generate your temporary image files without the need for an expensive clean up routine. Mind you though, if the site is only ever going to serve a small audience, is the IO performance hit really going to impact your application or system load greatly?