For one of the data sets on my graph, the legend should show a solid-color box, but it looks like there is a white line at the bottom.

The problem occurs only when the border of the bars has a width of half-point. Double-click on the legend box or the corresponding bar on the graph to open the Format Graph dialog. Change to no border, or a 1 pt. border, and the problem will go away.

This is the same legend with no border.