Filling under the a curve with ggplot graphs

Question

I would like to create a graph with the normal function from x=-2 to x=2 filled under the curve from -2 to 0. I've tried with ggplot2

qplot(c(-2, 2), stat="function", fun=dnorm, geom="line") +
+ geom_area(aes(xlim=c(-2,0)),stat="function", fun=dnorm)

But I get this graph completely filled instead (the black colour)

How can I get a plot filled only from -2 to 0?

Other options or packages are welcome.

I've also tried with only one command with ggplot and filled option but I can't get it either.
I know some people does it using polygons but the result is not so soft and nice.

PD: I repeat, the solution I'm looking for involves not generating x,y coordinates beforehand but using directly the function with stat="function", fun=dnorm or similar. Thus, my question is not a duplicate.

I've also tried

ggplot(NULL,aes(x=c(-2,2))) +  geom_area(aes(x=c(-2,0)),stat="function", fun=dnorm, fill="red") +
geom_area(aes(x=c(0,2)),stat="function", fun=dnorm, fill="blue")

But again it fills all the curve with a single color, blue. The red half seems to be overwritten. The same with geom_ribbon and other options.

score 26 · Accepted Answer · answered Jan 20 '17 at 09:35

26

Try this:

ggplot(data.frame(x = c(-2, 2)), aes(x)) +
  stat_function(fun = dnorm) + 
  stat_function(fun = dnorm, 
                xlim = c(-2,0),
                geom = "area")

answered Jan 20 '17 at 09:35

W. Joel Schneider

1,711
13
14

1

I guess this wasn't available when I asked the question. – skan Jan 20 '17 at 20:40

Paulo E. Cardoso · Answer 2 · 2015-10-21T15:50:35.537

6

Can't you generate your distribution data with dnorm instead?

library(ggplot2)
x<-seq(-2,2, 0.01)
y<-dnorm(x,0,1)
xddf <- data.frame(x=x,y=y)
qplot(x,y,data=xddf,geom="line")+
  geom_ribbon(data=subset(xddf ,x>-2 & x<0),aes(ymax=y),ymin=0,
              fill="red",colour=NA,alpha=0.5)+
  scale_y_continuous(limits=c(0, .4))

edited Oct 21 '15 at 15:50

answered Oct 20 '15 at 19:04

Paulo E. Cardoso

5,778
32
42

But I mean using stat="function" or similar, without generating previously x,y pairs. I just need to modify my geom_area() function with some parameter – skan Oct 20 '15 at 22:57
@skan I not sure how to achieve this with `geom_area`. `geom_ribbon()` seems to work just fine. – Paulo E. Cardoso Oct 20 '15 at 23:31

score 5 · Answer 3 · answered Jan 26 '22 at 16:52

These days, with after_stat() and after_scale(), you could also use a more flexible approach that lets you explicitly map ranges of x values to filled sections.

For example, filling some normal distribution quantiles:

library(ggplot2)

breaks <- qnorm(c(0, .05, .2, .5, .8, .95, 1))

ggplot(data.frame(x = c(-2, 2)), aes(x)) +
  scale_fill_brewer("x") +
  stat_function(
    n = 512,
    fun = dnorm,
    geom = "area",
    colour = "gray30",
    aes(
      fill = after_stat(x) |> cut(!!breaks),
      group = after_scale(fill)
    )
  )

This approach also works with other statistics, e.g. stat_density() for kernel density estimates:

set.seed(42)

ggplot(data.frame(x = rnorm(1000)), aes(x)) +
  scale_fill_brewer("x") +
  stat_density(
    n = 512,
    geom = "area",
    colour = "gray30",
    aes(
      fill = after_stat(x) |> cut(!!breaks),
      group = after_scale(fill)
    )
  )

Filling under the a curve with ggplot graphs

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