[R] ggplot scale_size - is there any way to specify breaks?

[Raj Minhas]

Hi,

I am plotting scores for students in a school using a command of the form

qplot(x=Subject, y=Student, data=dataset, geom="point", size=Score)

It lists the Subject factors on the x-axis and the Student factors on the
y-axis - dots proportional to the Score are plotted at the resultant grid.
It works just as I want. However, I want to plot a number of these graphs
(eg. datasets from different grades or school years) and want to keep the
dot sizes consistent across the various graphs. The default behavior of
scale_size() seems to be to train on the given dataset so if the range of
scores is different for different datasets then the dot sizes can be
inconsistent. For example, assume that the scores for the various subjects
can range from 1 to 4. Typically, the scores will have span the entire range
but it is not guaranteed. If none of the students in a grade have a score
lower than 2 on any of the subjects then the smallest dot size for this
grade will correspond to a score of 2 whereas the same dot size will
correspond to a score of 1 for the other grades.

Is there any way to specify a range to scale_size()? There is an option
called "breaks" in scale_size_identity() that does what I want. Is there
something similar for scale_size()?

Thanks.

-Raj.
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