I have the same issue. When viewing a poly-line chart on the screen, the screen image is usually acceptable. But, if I print it, the lines turn into hairlines and a lot of colors become very difficult to see.
A workaround I'm using is to click on the graph and copy the image. Then I paste it into a different application where the lines are heavier and easy to see. The disadvantage of this approach is that the lines are somewhat blurry, and I lose any other info which would have printed on the page outside the chart.