If this a FM11 native Chart, you can set the color scheme choices for the chart to as dark as possible and see if it makes any difference. If your primary use will be print, choose a chart color scheme that is darker than you want on screen but which may print better.
You might also try printing to PDF and then opening the PDF file. Saving to PDF tends to generate TIF/CYMK image-settings within the file which may darken the image. That's usually an unwanted modification, but it might make a little difference for you here.
Another thing you might try is making the chart as large as possible, perhaps on a separate layout.
I have a similar issue. When the line charts are saved as PDFs the lines are too fine. We need to be able to configure the thickness of the lines in the chart somehow. Is this possible in FileMaker Pro?