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How to calculate the area of ellipse drawn by ggplot2?


In ggplot2, after I drawing the ellipse plot using stat_ellipse, is there any way to calculate the area of this ellipse? Here is the code and the plot:

library(ggplot2)
set.seed(1234)
x <- rnorm (1:1000)
y <- rnorm (1:1000)
data <- cbind(x, y)
data <- as.data.frame(data)
ggplot (data, aes (x = x, y = y))+
  geom_point()+
  stat_ellipse()

enter image description here


Solution

  • You can calculate the area of the ellipse by finding its semi-major and semi-minor axes (as shown in this SO answer):

    # Plot object
    p = ggplot (data, aes (x = x, y = y))+
      geom_point()+
      stat_ellipse(segments=201) # Default is 51. We use a finer grid for more accurate area.
    
    # Get ellipse coordinates from plot
    pb = ggplot_build(p)
    el = pb$data[[2]][c("x","y")]
    
    # Center of ellipse
    ctr = MASS::cov.trob(el)$center  # Per @Roland's comment
    
    # Calculate distance to center from each point on the ellipse
    dist2center <- sqrt(rowSums((t(t(el)-ctr))^2))
    
    # Calculate area of ellipse from semi-major and semi-minor axes. 
    # These are, respectively, the largest and smallest values of dist2center. 
    pi*min(dist2center)*max(dist2center)
    
    [1] 13.82067