How to keep equal relative sizes of points in a graph?

Question

I want to make a graph where the size of the circles indicate the size of the sample. if i use plot in p1(), it works fine.

but if i try to have the different type of points colored, then the relative size is wrong.

How would I get both the red and green circles to be the same size?

p1<-function() {
    plot(t$x,t$y,cex=100*t$size,xlim=c(0,1),ylim=c(0.,1.))    
}
p2<-function() {
    plot(t$x[t$r=="0"],t$y[t$r=="0"],xlim=c(0,1),ylim=c(0.,1.),cex=100*t$size,col="red")
    points(t$x[t$r=="1"],t$y[t$r=="1"],xlim=c(0,1),ylim=c(0.,1.),cex=100*t$size,col="green")
}
l<-20
x<-seq(0,1,1/l)
y<-sqrt(x)
r=round(runif(n=length(x),min=0,max=.8))
n<-1:length(x)
size=n/sum(n)
t<-data.frame(x,y,r,n,size)
t$r<-factor(r)
str(t)
p1()

Answer 1

You have to change function p2 a bit. You are using t$size, all of it, when you should be subsetting by the factor t$r, since you are doing so when plotting the points.

If you plot t$x[t$r == "0"] versus t$y[t$r == "0"] then you must use the sizes corresponding to those points, which are t$size[t$r == "0"]. Alternatively, you could subset the data frame t first, and then use those two resulting data frames to plot the points. See function p2_alt at the end.

p2 <- function() {
  plot(t$x[t$r == "0"], t$y[t$r == "0"],
       xlim = c(0, 1), ylim = c(0., 1.),
       cex = 100*t$size[t$r == "0"],
       col = "red",
       xlab = "x", ylab = "y")
  points(t$x[t$r == "1"],
         t$y[t$r == "1"],
         xlim = c(0, 1), ylim = c(0., 1.),
         cex = 100*t$size[t$r == "1"],
         col = "green")
}

set.seed(651)    # make the results reproducible

l <- 20
x <- seq(0, 1, 1/l)
y <- sqrt(x)
r <- round(runif(n = length(x), min = 0, max = 0.8))
n <- 1:length(x)
size <- n/sum(n)
t <- data.frame(x, y, r, n, size)
t$r <- factor(r)
#str(t)
#p1()
p2()

p2_alt <- function() {
  df1 <- subset(t, r == "0")
  df2 <- subset(t, r == "1")
  plot(df1$x, df1$y,
       xlim = c(0, 1), ylim = c(0., 1.),
       cex = 100*df1$size,
       col = "red",
       xlab = "x", ylab = "y")
  points(df2$x,
         df2$y,
         xlim = c(0, 1), ylim = c(0., 1.),
         cex = 100*df2$size,
         col = "green")
}

p2_alt()

The graph is exactly the same, but maybe the code is more readable.

Finally, note that I have added arguments xlab and ylab to both p2() and p2_alt().