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I want to set background colors in my ggplot to highlight ranges of data. Particularly, I want to highlight [-0.1,0.1] with, say, green,[-0.25,-0.1) and (0.1,0.25] with orange. In other words, what I need are bars with some alpha-transparency whose y-limits are the plot's y range, and x-limits are set by me.

Ideally, I would want something that would not be sensitive to coord_cartesian(...) (as setting vline(...,size = X) would). Additionally, it would be nice to have something independent from any data, and based solely on plot coordinates. I tried geom_segment, but I could not figure it our how to set a width that would work.

library(ggplot2) x <- c(seq(-1, 1, by = .001)) y <- rnorm(length(x)) df <- as.data.frame(x=x,y=y) ggplot(df,aes(x,y)) + geom_point(aes(y*abs(x)),alpha=.2,size=5) + theme_bw() + coord_cartesian(xlim = c(-.5,.5),ylim=c(-1,1))

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2 Answers 2

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You can add the "bars" with geom_rect() and setting ymin and ymax values to -Inf and Inf. But according to @sc_evens answer to this question you have to move data and aes() to geom_point() and leave ggplot() empty to ensure that alpha= of geom_rect() works as expected.

ggplot()+ geom_point(data=df,aes(x=y*abs(x),y=y),alpha=.2,size=5) + geom_rect(aes(xmin=-0.1,xmax=0.1,ymin=-Inf,ymax=Inf),alpha=0.1,fill="green")+ geom_rect(aes(xmin=-0.25,xmax=-0.1,ymin=-Inf,ymax=Inf),alpha=0.1,fill="orange")+ geom_rect(aes(xmin=0.1,xmax=0.25,ymin=-Inf,ymax=Inf),alpha=0.2,fill="orange")+ theme_bw() + coord_cartesian(xlim = c(-.5,.5),ylim=c(-1,1))

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I just want to check with you: if you set one of the "orange" alpha to e.g. 0.1, and the other to 0.9, does it change the appearance of the rectangles? I.e. does alpha work the way at least I expected in geom_rect? Sorry in advance if I am doing something wrong here.
@Henrik Thank you for pointing to problem with alpha - updated my answer.
I had a vague feeling I had been struggling with geom_rect and alpha previously. Just before I got crazy then, I gave up and went for annotate instead. Thanks a lot @Didzis Elferts for clarifying this 'issue'!
I had the same issue before. Thanks for the clarification. I will set this as the answer because Didzis posted before Henrik. Thanks both.
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You may try annotate, which takes vectors of xmin and xmax values.

ggplot(df,aes(x,y)) + geom_point(aes(y*abs(x)), alpha =.2, size = 5) + annotate("rect", xmin = c(-0.1, -0.25, 0.1), xmax = c(0.1, -0.1, 0.25), ymin = -1, ymax = 1, alpha = 0.2, fill = c("green", "orange", "orange")) + theme_bw() + coord_cartesian(xlim = c(-.5,.5),ylim=c(-1,1))

enter image description here

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