Shading in prediction intervals

I have the following code for the minimum and maximum of my prediction interval

y.down=lines(x[x.order], set1.pred[,2][x.order], col=109)
y.up=lines(x[x.order], set1.pred[,3][x.order], col=109)

y <- lines(1:3, 1:3)
y

domain=min(x):max(x)

polygon(c(domain,rev(domain)),c(y.up,rev(y.down)),col=109)

It doesnt seem to shade the right region, it gives me a trapezoid.

Any help? Thanks!
		 	   		  
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