Hi lanredoyes, welcome to the forums
Ask the prof for a syllabus so you can plan ahead and get started on course reqs early. A lot less stress, believe me
Your form action points to itself. That could make for a big file that might be more difficult to work with, up to you.
The form looks pretty well done. I'm unfamilliar with either jpgraph or rpgraph but I imagine they're similar to the PEAR package I've used.
As Cups explained, the form's input values get sent to the handler upon submit.
The handler parses the values (and should sanitize and validate them) and passes them in a database query.
Code (somewhere, the handler?) then takes the returned values and passes them to the graph script.
The graph script generates an image and returns it.
I would use GET instead of POST, but again, up to you.
Where and how you display the graph image is also up to you, IMHO easiest would be in a window by itself.
As Cups also mentioned, do some var_dumps so you can see what's getting passed around rather than simply trying something and being puzzled if it "doesn't work", it will be a big time saver in the long run.