How to expand axis asymmetrically with ggplot2 without setting limits manually?

问题:

I want the bottom axis for a barplot to be zero, but top expanded by a small amount (according to some scale). However, I am plotting the output of stat_summary so I do not have the maximum value available to me, and I am using facets with scale="free_y" so I cannot manually set them. Otherwise I would use baptiste's trick of adding geom_blank. I know I could read off the values and create a data frame with upper bounds manually, but can this be set without having to hard-code the limits?

The other option is to compute the means outside of the function call and just plot that without calling stat_summary (this way I have the upper bounds directly for each facet), but is there any way around this?

Example:

ggplot(mtcars)+   stat_summary(aes(cyl,mpg),                fun.y="mean",                geom="bar")+   scale_y_continuous(expand=c(0,0))+   facet_grid(carb~.,"free_y") 

回答1:

You can "extend" ggplot by creating a scale with a custom class and implementing the internal S3 method scale_dimension like so:

library("scales") scale_dimension.custom_expand 

This is also an example of overriding the default scale. Now, to get the desired result you specify expand like so

qplot(1:10, 1:10) + scale_y_continuous(expand=list(c(0,0.1), c(0,0))) 

where the first vector in the list is the multiplicative factor, the second is the additive part and the first (second) element of each vector corresponds to lower (upper) boundary.

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