ggplotd Fixed ratio between x and y axes

[kerdemdemir]

How can I configure a fixed ratio between x and y axes in ggplotd 
?

I easily found what I am looking for in ggplot which ggplotd 
inspires a lot.
http://www.cookbook-r.com/Graphs/Axes_(ggplot2)/

But ggplotd documentation does not have any info about that. Even 
I go through the source code spend around half a hour I couldn't 
figure out how to achieve:

sp <- ggplot(dat, aes(xval, yval)) + geom_point()
sp + coord_fixed(ratio=1/3)

Erdem