You want to change the appearance of the lines in a line graph.

The type of line (solid, dashed, dotted, etc.) is set with linetype, the thickness (in mm) with size, and the color of the line with colour.

These properties can be set (as shown in Figure 4-11) by passing them values in the call to geom_line():

ggplot(BOD, aes(x=Time, y=demand)) +
    geom_line(linetype="dashed", size=1, colour="blue")

Figure 4-11. Line graph with custom linetype, size, and colour

If there is more than one line, setting the aesthetic properties will affect all of the lines. On the other hand, mapping variables to the properties, as we saw in Making a Line Graph with Multiple Lines, will result in each line looking different. The default colors aren’t the most appealing, so you may want to use a different palette, as shown in Figure 4-12, by using scale_colour_brewer() or scale_colour_manual():