Next GD headache

Short question: I can't find this in the docs anywhere. Is it possible to register more than one session variable at a time? If so, what is the syntax? Long question: I am trying to generate a bar chart to accompany some figures I calculate. I have the code to generate the bar chart in a separate page, and I call the script as an image. So far so good. The thing I can't figure out is how best to pass variables to the graphic code. I thought about setting them as globals, but I thought that might be problematic in some way. The only other idea I had was to set them as session variables -- and that works, but it strikes me as (i dunno) inelegant??? Suggestions anyone?