shading certain area in graph - a combination of lines and point plots in base R

Question

I have following question:

set.seed(1234)
dis = seq(1, 800, 10)
 yvar = rnorm(length (dis), 50, 10)
myd <- data.frame (xvar = dis, yvar = yvar, yvarL = 
yvar - rnorm (length (yvar),5, 1), yvarU = yvar + rnorm (length (yvar), 5, 1))
plot(myd$xvar,myd$yvar,pch=19,cex=.75,col="blue")
points(myd$xvar,myd$yvarL,col="gray", pch=19,cex=.5)
points(myd$xvar,myd$yvarU,col="gray", pch=19,cex=.5)

Y has three values - Y, upper and lower confidence. I would like to shade between two intervals in axis. Rest of graph would be point only. The hypothesized product graph should be similar (not exactly).

How can I shade and produce line plot between two intervals say between 650 to 690 and 210 to 280 in myd$xvar. I tried to do some polygot fitting based on this post ( shading within xy curve plot in R), but not successful.

polygon(c(myd$xvar[1:200], myd$xvar[200:1]), c(myd$xvar[1:200],
   myd$yvar[200:1]), col="gray")

Answer 1

One problem with your data is that myd has only 80 observations, but you are trying to plot 200... Just feed the x variable twice into polygon() (once as-is, once in reverse order), then the y variable (first the lower limit as-is, then the upper limit in reverse order). Then you have your shading:

polygon(c(myd$xvar[20:40], myd$xvar[40:20]),c(myd$yvarL[20:40],
 myd$yvarU[40:20]), col="gray")

However, the shading will cover some points, so you will have to re-plot them:

points(myd$xvar,myd$yvar,pch=19,cex=.75,col="blue")
points(myd$xvar,myd$yvarL,col="gray", pch=19,cex=.5)
points(myd$xvar,myd$yvarU,col="gray", pch=19,cex=.5)

EDIT: rfan below asks to shade based on the x coordinate, not the index, and to add a line in the shaded region. For this, it makes sense to extract an index of the relevant points (we use that myd$xvar is sorted):

index <- which(myd$xvar>=200 & myd$xvar<=400)

We then plot based on this index, which we can easily modify if we need to:

polygon(c(myd$xvar[index], myd$xvar[rev(index)]),c(myd$yvarL[index], 
 myd$yvarU[rev(index)]), col="gray")

Now, add a line and the additional points:

lines(myd$xvar[index],myd$yvar[index],pch=19,cex=.75,col="blue",type="o")
points(myd$xvar,myd$yvarL,col="gray", pch=19,cex=.5)
points(myd$xvar,myd$yvarU,col="gray", pch=19,cex=.5)