ggplot2: How to shade an area above a function curve and below a line?

Question

So I have a dataframe like this:

a_data <-
  data.frame(
    f = f,
    alpha = alpha,
    asymptote = alpha_1_est)

and a function like this:

a_formula <- function(x) {
  0.7208959 - 0.8049132 * exp(-21.0274 * x)}

I use them with ggplot2:

ggplot(a_data, aes(x = f, y = alpha)) + 

geom_point() +

#function curve
stat_function(fun = a_formula,
              color = "red") +

#asymptote of alpha
geom_hline(
  yintercept = asymptote,
  linetype = "longdash",
  color = "blue")

which yields a plot like this:

what I want and can't find a way to do is to shade the area between the y axis, the function curve (red) and the asymptote line (dashed), like this:

I have tried to squeeze a ribbon or a polygon in there, but it doesn't work correctly - maybe it's because I want to shade above the curve, not below (below works just fine).

This is how the dataframe looks like:

> head(a_data)
     f       alpha asymptote
1 0.01 0.007246302 0.7208959
2 0.03 0.374720198 0.7208959
3 0.05 0.484362949 0.7208959
4 0.07 0.540090209 0.7208959
5 0.09 0.625383303 0.7208959
6 0.11 0.590898201 0.7208959

P.S. I am fairly new to stackoverflowing, so if I broke any convention or otherwise messed up the question, don't hesitate to point it out.

Answer 1

The example below shows how geom_ribbon can be conveniently used for coloring the area between the horizontal line and the curve.

df1 <- structure(list(x = c(0.01, 0.03, 0.05, 0.07, 0.09, 0.11), y = c(0.007246302, 
0.374720198, 0.484362949, 0.540090209, 0.625383303, 0.590898201
), asymptote = c(0.7208959, 0.7208959, 0.7208959, 0.7208959, 
0.7208959, 0.7208959)), .Names = c("x", "y", "asymptote"), class = "data.frame", row.names = c("1", 
"2", "3", "4", "5", "6"))

a_formula <- function(x) { 0.7208959 - 0.8049132*exp(-21.0274*x) }

xs <- seq(min(df1$x),max(df1$x),length.out=100)
ysmax <- rep(0.7208959, length(xs))
ysmin <- a_formula(xs)
df2 <- data.frame(xs, ysmin, ysmax)

library(ggplot2)
ggplot(data=df1) + geom_point(aes(x=x, y=y)) +
geom_line(aes(x=x, y=asymptote), lty=2, col="blue", lwd=1) +
stat_function(fun = a_formula, color="red", lwd=1) +
geom_ribbon(aes(x=xs, ymin=ysmin, ymax=ysmax), data=df2, fill="#BB000033")