[R] Ggplot barchart drops factor levels: how to show them with zero counts?

[Bart6114]

Hello,

When plotting a barchart with ggplot it drops the levels of the factor for
which no counts are available.

For example:

library(ggplot)
mtcars$cyl<-factor(mtcars$cyl)
ggplot(mtcars[!mtcars$cyl==4,], aes(cyl))+geom_bar()
levels(mtcars[!mtcars$cyl==4,])

This shows my problem. Because no counts are available for factorlevel '4',
the label 4 dissapears from the plot. However, I would still like it to show
up, but without a bar (zero observations).

I would like to use this for the presentation of data with a Likert-like
scale.

Thanks in advance!
Bart

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