Graphic legend includes empty categories?

[Andrea Spray]

Greetings!
Is there a way to force a pie graph legend to include categories that are empty?
For example, each category below is listed with the number of associated observations:
cat1 4
cat2 22
cat3 21
cat4 0

I want cat4 to still appear in the legend even though it is empty.
Thanks in advance!

[Nick Cox]

I got as far as showing an extra line in the legend() -- and how to do that is explicit in the help.

I failed to get another colour shown.

clear
input category freq
1 4
2 22
3 21
4 0
end

graph pie [fw=freq] , over(category) legend(order(1 2 3 - "pies: a bad idea")) ///
pie(4, color(magenta))

EDIT: The trick seems to be a slice so thin it won't be plotted, but you'll still see a legend entry.

clear
input category freq
1 4
2 22
3 21
4 1e-6
end

graph pie [aw=freq] , over(category) legend(order(1 2 3 4 "(not present)")) ///
pie(4, color(magenta))