ggbiplot: how to maintain group colors after changing point size?

Question

I am doing a PCA on plots in 2 habitat types in which I collected data on multiple environmental variables. I was able to change the colors of the points from the ggbiplot defaults. I want the size of each point to depend on canopy cover in that plot, and I was able to do that by:

point.size = df$canopy.cover * 0.1  

where df$canopy.cover values range from 0-100 and 0.1 because a canopy cover of 100%=point size 10.

The problem: I can't maintain the colors associated with the two groups after changing the size of points. Using the following pseudo-data:

env.vars<-data.frame(replicate(5,sample(0:10,20,rep=TRUE)))

cover<-c(89, 92, 72, 53, 88, 89, 71, 83, 71, 66, 23, 30,  5, 15, 57, 54,0, 23, 9, 16)

habitat<-c("habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2")

point.size<-cover*0.1

nest.env.pca <- prcomp(env.vars, center = TRUE, scale. = TRUE) 

g <- ggbiplot(nest.env.pca, obs.scale = 1, var.scale = 1, 
     group = habitat, ellipse = TRUE, 
     circle = TRUE, varname.size=3)+
     scale_colour_manual(values=c("blue", "red")) +
     geom_point(size=point.size)

print(g)

I get something similar to:

When I replace:

geom_point(size=point.size)

with:

geom_point(aes(colour=habitat), size=point.size)

as per ggbiplot - change the point size I get the following following error:

Error: Incompatible lengths for set aesthetics: size

Any suggestions? Thanks.

EDIT: some PseudoData to try it out with:

Answer 1

This is a general ggplot2 syntax issue, I think, as it occurs with simpler examples (e.g., ggplot(mtcars) + geom_point(aes(disp, hp, color=cyl), size =mtcars$model))

But, I think this will work for you:

geom_point(aes(X1, X2, color=habitat, size = point.size)) + scale_size_identity()

So the full answer with the code in the question is (from Emilio's comment below):

env.vars<-data.frame(replicate(5,sample(0:10,20,rep=TRUE)))

cover<-c(89, 92, 72, 53, 88, 89, 71, 83, 71, 66, 23, 30,  5, 15, 57, 54,0, 23, 9, 16)

habitat<-c("habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2", "habitat1", "habitat2")

point.size<-cover*0.1

nest.env.pca <- prcomp(env.vars, center = TRUE, scale. = TRUE) 

g <- ggbiplot(nest.env.pca, obs.scale = 1, var.scale = 1, 
     group = habitat, ellipse = TRUE, 
     circle = TRUE, varname.size=3)+
     scale_colour_manual(values=c("blue", "red")) +
     geom_point(aes(color=habitat, size = point.size)) + scale_size_identity() 

print(g)