how to customize y-axis step size in R

Question

I would like to plot a scatterplot with y-axis is customized to step size of 0.2, within range of 0 - 2.6, and x-axis can be auto-defined. I tried the below, but it doesnt work. May I know how should I set the param correctly?

---

# Read data
pt.n <- read.table("p0_n300m20r1c1_regression.txt", header=T)
# auto-scale
# plot(pt.n$maee~pt.n$idx, main="P2PSim Seq#1,300n,20%,1r,Corrective", ylab="MAEE", xlab="Seq #")

# customize
ylabel <- c(0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6)
y_range <- range(0, ylabel)
plot(pt.n$maee~pt.n$idx, main="P2PSim Seq#3,300n,20%,1r,Corrective", ylab="MAEE", xlab="Seq #", ylim=y_range, axes=FALSE, ann=FALSE)
axis(1, at=0:6, lab=c(0,50,100,150,200,250,300))
axis(2, las=1, at=0.2*0:y_range[1])
box()

Answer 1

If something is not working check each bit of the thing that isn't doing what you want to make sure you are supplying the correct data and haven't made a booboo. If we run the bits of your code that are associated with the axis

ylabel <- c(0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, 2.2, 2.4, 2.6)
y_range <- range(0, ylabel)
0.2*0:y_range[1]

You would immediately see the problem:

R> 0.2*0:y_range[1]
[1] 0

where you are basically telling R to draw a tick at 0. Even if you chose the correct element of y_range (the maximum is in the second element) you still wouldn't get the right answer:

R> 0.2*0:y_range[2]
[1] 0.0 0.2 0.4
R> 0:y_range[2]
[1] 0 1 2

and that is because of the way the : operator works. A call of x:y is essentially a call to seq(from = x, to = y, by = 1) and because 2.6+1 is greater than 2.6 (the to argument) R creates the sequence 0, 1, 2.

If you want to draw ticks and label at 0 - 2.6 incrementing by 0.2 then use:

ylabel <- seq(0, 2.6, by = 0.2)
axis(2, at = ylabel)

where ylabel now contains:

R> ylabel
[1] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

To illustrate:

dat <- data.frame(y = runif(20, min = 0, max = 3),
                  x = rnorm(20))

plot(y ~ x, data = dat, axes = FALSE)
ylabel <- seq(0, 2.6, by = 0.2)
axis(1)
axis(2, at = ylabel, las = 1)
box()

which produces