It's a few months later, but here's how:

library(ggplot2)

qplot(mpg, wt, data=mtcars, shape=cut(qsec, breaks=10), size=I(5)) +

scale_shape_manual(values=LETTERS[1:10]) +

labs(shape = "qsec")

Essentially, you need to provide a factor vector (of the same length as the plotted data) to the argument "shape" in the qplot call. In my example, I've just created a factor from the qsec column of the mtcars dataframe. You then add the call to scale_shape_manual, where you provide the plotting characters you'd like to associate with each level of your factor. (Note that the vector of plotting characters can be longer than the number of unique factor levels present in your data, in which case only the first nlevels elements will be used.) My use of "size" and labs() were purely for aesthetics, and not entirely relevant to your question.

The equivalent with ggplot():

ggplot() + geom_point(data=mtcars, mapping=aes(wt, mpg, shape=cut(qsec, breaks=10)), size=5) +

scale_shape_manual(values=LETTERS[1:10]) +

labs(shape="qsec")

It's a few months later, but here's how:

library(ggplot2)

qplot(mpg, wt, data=mtcars, shape=cut(qsec, breaks=10), size=I(5)) +

scale_shape_manual(values=LETTERS[1:10]) +

labs(shape = "qsec")

Essentially, you need to provide a factor vector (of the same length as the plotted data) to the argument "shape" in the qplot call. In my example, I've just created a factor from the qsec column of the mtcars dataframe. You then add the call to scale_shape_manual, where you provide the plotting characters you'd like to associate with each level of your factor. (Note that the vector of plotting characters can be longer than the number of unique factor levels present in your data, in which case only the first nlevels elements will be used.) My use of "size" and labs() were purely for aesthetics, and not entirely relevant to your question.

The equivalent with ggplot():

ggplot() + geom_point(data=mtcars, mapping=aes(wt, mpg, shape=cut(qsec, breaks=10)), size=5) +

scale_shape_manual(values=LETTERS[1:10]) +

labs(shape="qsec")

+ scale_shape_manual(values=seq(0,17))+