I see that I didn't include enough information in my question -- sorry (I'm a bit new). We are using a networked Linux envrionment so I won't have to worry about different platforms or machines. The program is not a client, just a standalone that currently accepts file input. I wanted to try to go the extra mile and add a GUI.
A web-based (unfortunately) will not be the best solution here. I know that I need a graphics package and really just wanted to know if the Monks had a favorite package. I don't know the advantages/disadvantages of any, so I was trying to save myself some time in investigating that.