The interior decoration of graphics generate a lot of ink that does not tell the viewer anything new. The purpose of decoration varies – to make the graphic appear more scientific and precise, to enliven the display, to give the designer an opportunity to exercise artistic skills. Regardless of its cause, it is all non-data-ink or redundant data-ink, and it is often chartjunk.
Remove the grid (or use a light gray grid) and non-essential non-data elements. For exmple, avoid using shadow. Use a white background for your chart, when possible. Reduce non-data graphic elements (e.g. reduce the thickness of the bars in a bar chart).
The Darkhorse Analytics is another great resource to learn how to remove chartjunk.