See the post "Resize smaller" from Nov. 2003. You can not make the graph so small that the plotting are goes to 0, that will create an overflow condition.
If you are creating extremely small graphs, and then trying to print them full size to the printer, that could be why your lines are getting so thick when you print. The entire graph will be rescaled to the printer, and this will make the lines and text proportionally thicker and larger.
The graph should be sized similar on the screen as to the printer to maintain the WYSIWYG.