Shape scales from Tremmel (1995)

(ggplot(mtcars, aes(x=mpg, y=hp, shape=factor(cyl)))
 + geom_point() + scale_shape_tremmel())

(ggplot(mtcars, aes(x=mpg, y=hp, shape=factor(cyl)))
 + geom_point() + scale_shape_tremmel(n3alt=FALSE))

(ggplot(mtcars, aes(x=mpg, y=hp, shape=factor(am)))
 + geom_point() + scale_shape_tremmel())

(ggplot(mtcars, aes(x=mpg, y=hp, shape=factor(am)))
 + geom_point() + scale_shape_tremmel(overlap=TRUE))