How to adjust point size to graph scale in ggplot2?

Question

Let some data be generated:

x <- -10*cos(seq(0, pi, length.out = 100))+1 y <- 10*seq(0, pi, length.out = 100) xerr <- rep(2, 100) yerr <- rep(2, 100) dd <- as.data.frame(cbind(x, y, xerr, yerr)) 

Here I have x and y coordinates of some points with their errors, xerr and yerr (for convenience, I stood them). I would like to introduce these errors with the size of the dots. This is easy to do:

 ggplot() + geom_point(data = dd, aes(x, y, size = sqrt(xerr^2 + yerr^2)), colour = "gray") + geom_path(data = dd, aes(x, y), colour = "red", size = .5) + scale_size_identity() + theme_bw() 

However, the size of these points is determined on a scale that has nothing to do with the scale of the graph. Is there a way to adjust the size of the dots in relation to the scale of the plot? In the above example, the radius of each point should have a size equal to 2.828 and not less than one, as it is now.

Answer 1

One way is to explicitly draw ellipses with axes determined by the size of the errors.

 x <- -10*cos(seq(0, pi, length.out = 10))+1 y <- 10*seq(0, pi, length.out = 10) xerr <- runif(10, 1, 5) yerr <- runif(10, 1, 5) dd <- as.data.frame(cbind(x, y, xerr, yerr)) dd$frame <- factor(seq(1:10)) 

For this purpose, we define our function for generating ellipses:

 ellipseFun <- function(center = c(0, 0), axes = c(1, 1), npoints = 101){ tt <- seq(0,2*pi, length.out = npoints) xx <- center[1] + axes[1] * cos(tt) yy <- center[2] + axes[2] * sin(tt) return(data.frame(x = xx, y = yy)) } 

Then we create matrices for all ellipses:

 ddEll <- data.frame() for(k in levels(dd$frame)){ ddEll <- rbind(ddEll, cbind(as.data.frame(with(dd[dd$frame == k,], ellipseFun(center = c(x, y), axes = c(xerr, yerr), npoints = 101))),frame = k)) } 

And finally, we can build them:

 library(ggplot2) ggplot() + geom_point(data = dd, aes(x, y)) + geom_polygon(data=ddEll, aes(x = x, y = y, group = frame), colour = "gray", fill = "red", alpha = .2) + scale_size_identity() + theme_bw() + xlim(c(-20, 20)) + ylim(c(-5, 35)) + coord_fixed()