ggplot2 - Shade area above line

Question

I have some data that is constrained below a 1:1 line. I would to demonstrate this on a plot by lightly shading the area ABOVE the line, to draw the attention of the viewer

Answer 1

As far as I know there is no other way other than creating a polygon with alpha-blended fill. For example:

df <- data.frame(x=1, y=1)
df_poly <- data.frame(
    x=c(-Inf, Inf, -Inf),
    y=c(-Inf, Inf, Inf)
)

ggplot(df, aes(x, y)) + 
    geom_blank() + 
    geom_abline(slope=1, intercept=0) + 
    geom_polygon(data=df_poly, aes(x, y), fill="blue", alpha=0.2) +