stacking transparent images
here's what i would like to do - but don't know how to do:
against a non-transparent image (it could be a bg image, or not), place a series of transparent images one at a time in a stack. effect would be to show a cumulative result.
my particular problem: against a map of the Atlantic, show ship voyages during the eighteenth century. i have all the data i need, and i have the transparent images for each year. now i want to let the user select start/end years and watch the voyages accumulate. if anyone cares, i used the open source stat package R, and the data available from the CLIWOC project.