Excel uses lines for axes, line charts, and trendlines. It also uses lines to create borders around a variety of chart elements—the chart area, the plot area, the titles, the legends, the markers on bar and column charts, and so on. Not all of these borders appear by default (for example, chart titles are borderless by default), but all are available if you want them.

The formatting options for all the lines and borders are essentially the same. You can color them, fatten them, change them from solid to dotted or dashed, and so on. (You can also delete them.)

For example, suppose you want to change the color of a series line on a line chart. To get to the formatting dialog box, double-click a member of the series. In the Format Data Series dialog box (see Figure 21-2), select the Line Color category or the Line Style category. (If you were customizing a border instead of a line, the names of these categories would be Border Color and Border Styles.)

The default option in the Line Color category is Automatic—which means “let Excel decide.” Usually, that’s a good choice, but you have alternatives. If you select Solid Line, the dialog box changes to include a color picker and a Transparency slider. (The default Transparency setting is 0, which means opaque; if your chart has an interesting graphic background, you might want to experiment with making a line element partially transparent.) If you select Gradient Line, several new options appear. For more information about creating gradient lines, see Filling an Area with a Color Gradient on page 702. The procedures for creating gradient lines are essentially the same as for areas.

Figure 21-2. When you select Line Color or Line Style, you’ll see essentially the same set of options for all the line elements in Excel charts. A similar set of options is available for borders.

The Solid Line and Gradient Line options have to do with the line’s color, not its weight and style. Solid Line means the entire line is one color, whether or not the line is broken up into dashes or dots. The Gradient Line option—more commonly used for areas than for lines—lets you apply multiple colors with smooth transitions.

When you select the Line Style category, you can indicate your choice of simple or compound line and unbroken or broken. As the following illustrations show, the Compound Type drop-down list offers four alternatives to a simple line, and the Dash Type drop-down list lets you chop your line in seven different fashions. Whatever you select or don’t select in these two drop-down lists, you can vary the weight of your line via the Width drop-down list.