Due to compatibility with some very early versions of ChartDirector, by default, the legend icon of all entries will be a square. To use line style icons, you need to add the line:
LegendBox b = c.getLegend();
With setLineStyleKey, lines will use line icons. Other layer types will use the icons appropriate for them (box icons for bar/area, symbols for scatter layers, etc). To add a line style icon, just provide a non-zero line width in LegendBox.addKey:
b.addKey("ABC", 0x336699, 2);
ChartDirector will try to keep all icons the same width. This ensures the text for different icon types will align properly for vertical legend boxes.