Fill area under the density curves in R

Fill area under the density curve

The polygon function can be used to shade the area under the density curve. You just need to pass the density object to it and specify a color.

# Data
set.seed(3)
y <- rnorm(150)

den <- density(y)

# Plot
plot(den)

# Fill area
polygon(den, col = "slateblue1")

Fill area under density curve in R

Fill density area with transparency

If you are plotting several density curves and some of them overlap it is recommended to use colors with transparency, so you will be able to see both curves. For this purpose you can use the rgb function and specify the transparency (between 0 and 1) in the alpha argument. See colors section if you need to convert HEX colors to RGB.

Density plot with shaded area with transparency

# Data
set.seed(3)
x <- rnorm(200)
y <- rnorm(150, mean = 1)

# Density estimations
denx <- density(x)
deny <- density(y)

# Plot
plot(denx,
     ylim = c(0, max(c(denx$y, deny$y))),
     xlim = c(min(c(denx$x, deny$x)),
              max(c(denx$x, deny$x))))
lines(deny)

# Fill the areas
polygon(denx, col = rgb(0.78, 0.89, 1, alpha = 0.6))
polygon(deny, col = rgb(0.51, 0.44, 1, alpha = 0.6))

Fill specific area under the density curve

You can also fill some specific areas under the curves, such as the area greater than a specific point, the area lower than a point or the area between two points.

Fill area for values greater than 1

# Data
set.seed(3)
y <- rnorm(150)

# Density estimation
den <- density(y)

# Plot
plot(den)

# Fill area for values greater or equal to 1
value <- 1

polygon(c(den$x[den$x >= value ], value),
        c(den$y[den$x >= value ], 0),
        col = "slateblue1",
        border = 1)

Shade an specific area of a density curve in R

Fill area for values lower than 1

# Data
set.seed(3)
y <- rnorm(150)

# Density estimation
den <- density(y)

# Plot
plot(den)

# Fill area for values lower or equal to 1
value <- 1

polygon(c(den$x[den$x >= value ], value),
        c(den$y[den$x >= value ], 0),
        col = "slateblue1",
        border = 1)

Shade an area of the density curve for values lower than 1

Fill area for values between -1 and 1

# Data
set.seed(3)
y <- rnorm(150)

# Density estimation
den <- density(y)

# Plot
plot(den)

# Area between -1 and 1
value1 <- -1
value2 <- 1

# Lower and higher indices on the X-axis
l <- min(which(den$x >= value1))
h <- max(which(den$x < value2))

polygon(c(den$x[c(l, l:h, h)]),
        c(0, den$y[l:h], 0),
        col = "slateblue1")

Fill specific area of the density curve