"lines()" acting like "polygon()" R?

Question

I'm trying to draw two black lines around a red regression line. But the lines() command draws something more like a polygon() than a simple line (see picture below code).

I'm wondering is there a fix to simply draw two lines around the regression line (i.e., uncertainty intervals), or I'm missing something?

library(rstanarm) 
data(kidiq)
d <- kidiq  

fit <- stan_glm(kid_score ~ mom_iq,
           data = d,   
           prior = normal(0, 2.5),  
           prior_intercept = normal(0, 10),  
           prior_aux = cauchy(0, 100)) 

plot(kid_score ~ mom_iq, data = d, type = "n")
abline(fit, col = 2)

pred_lin <- posterior_linpred(fit)

loop <- length(d$mom_iq)
I <- matrix(NA, loop, 2)
for(i in 1:loop){
I[i,] = quantile(pred_lin[,i], c(.025, .975))
}
lines(d$mom_iq, I[,1], lty = 2)
lines(d$mom_iq, I[,2])

Answer 1

Try the ordered data.frame like:

a <- cbind(d$mom_iq, I[,1])
a <- a[order(a[,1]),]
lines(a)

So you can also write:

lines(sort(d$mom_iq), I[,2][order(d$mom_iq)])

or simply:

apply(I, 2, function(x) lines(sort(d$mom_iq), x[order(d$mom_iq)]))