Odd behavior of mtext in R - r

Question

I am trying to design a fairly customized plot in R.
One thing I want to do is add tick marks that are different from the labels - that is, only every 5th tick mark will be labeled. I didn't see an easy way to do this, so I did this:
plot(x = Freq_, y = Mean_ipsi,
pch = 20,
ylim = c(-0.5, .9),
col = 1 + (ProbF < .05) + (ProbF < .01),
xaxt = 'n',
xlab = "Frequency (MHz)", ylab = "z-in minus z-out",
main = "Temporal, Engle 1, Epi, subjectwise",
yaxt = 'n')
mtext(text = seq(1.56, 35.1, by = 1.95),
side = 1, at = seq(1.56, 35.1, by = 1.95), cex = .5,line = 0.25)
axis(1, at = Freq_, tick = TRUE, labels = NA)
and it worked just as I wanted.
But when I changed some of the code preceding mtext I got unexpected results
plot(x = Freq_, y = Mean_ipsi,
pch = 20,
ylim = c(-0.5, .9),
col = "red",
xlab = "Frequency (MHz)", ylab = "z-in minus z-out",
main = "Temporal, Engle 1, Epi, subjectwise
\n p values for difference between ipsi and contra",
yaxt = 'n', type = 'o')
mtext(text = seq(1.56, 35.1, by = 1.95),
side = 1, at = seq(1.56, 35.1, by = 1.95),
cex = .5,line = 0.25)
axis(1, at = Freq_, tick = TRUE, labels = NA)
Now, in addition to the x-axis being labeled with every numbers 1.56, 3.51, etc. I get large numbers (cex = 1, I think) at 5, 10 and so on. I don't want these.
I have no idea what is happening here.

Answer 1

You're missing xaxt="n" in your second version.
Freq_ <- seq(1.56, 35.1, by = 1.95)
Mean_ipsi <- (0.01 * Freq_)
ProbF <- 0.0
#First Version
plot(x = Freq_, y = Mean_ipsi,
pch = 20,
ylim = c(-0.5, .9),
col = 1 + (ProbF < .05) + (ProbF < .01),
xaxt = 'n',
xlab = "Frequency (MHz)", ylab = "z-in minus z-out",
main = "Temporal, Engle 1, Epi, subjectwise",
yaxt = 'n')
mtext(text = seq(1.56, 35.1, by = 1.95),
side = 1, at = seq(1.56, 35.1, by = 1.95), cex = .5,line = 0.25)
axis(1, at = Freq_, tick = TRUE, labels = NA)
#=============================================
#Second Version
plot(x = Freq_, y = Mean_ipsi,
pch = 20,
ylim = c(-0.5, .9),
col = "red",
xlab = "Frequency (MHz)", ylab = "z-in minus z-out",
main = "Temporal, Engle 1, Epi, subjectwise
\n p values for difference between ipsi and contra",
yaxt = 'n', type = 'o') ##### <----- add xaxt="n" here #####
mtext(text = seq(1.56, 35.1, by = 1.95),
side = 1, at = seq(1.56, 35.1, by = 1.95),
cex = .5,line = 0.25)
axis(1, at = Freq_, tick = TRUE, labels = NA)