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

In geom_point(), set the size, shape, colour, and/or fill outside of aes() (the result is shown in Figure 4-14):

ggplot(BOD, aes(x=Time, y=demand)) +
    geom_line() +
    geom_point(size=4, shape=22, colour="darkred", fill="pink")

Figure 4-14. Points with custom size, shape, color, and fill

The default shape for points is a solid circle, the default size is 2, and the default colour is "black". The fill color is relevant only for some point shapes (numbered 21–25), which have separate outline and fill colors (see Using Different Point Shapes for a chart of shapes). The fill color is typically NA, or empty; you can fill it with white to get hollow-looking circles, as shown in Figure 4-15: