What is it that you mean by "use last successful plot settings"? I ask as i'm writing a plot utility for my company now, and the way i am doing is that users pick the appropriate details from a form (plotter, paper size, plotstyle, number of copies etc), then pick the plot command button, which first saves the details of what was selected on the form to the registry, and then runs the plotting. When they go back to it, the program then uses the settings stored in the registry to populate the controls on the form.
Does this answer your question?
