[R] plotting a grid with grid() ?

[John Kane]

I am trying to come up with a way of shading-in a grid for a simple pattern 

So far I can draw a square where I want but I cannot seem to draw a complete grid. I am just drawing them along the diagonal!! 

Clearly I am missing something simple but what?

Any suggestions gratefully accepted.

Example
#====================================================================
op <-  par(xaxs="i", yaxs="i")
plot(c(1, 11), c(1,11), type ="n", xlab="", ylab="")

x1 <- rep(1:10, each=10)
x2 <- rep(2:11, each=10)
y1 <- rep(1:10, each=10)
y2 <- rep(2:11, each=10)

# no grid :(
rect(x1,y1,x2,y2, border="blue")

rect(2,2,3,3, col="red")

x1 <- rep(1:10,10)
x2 <- rep(2:11, 10)
y1 <- rep(1:10, 10)
y2 <- rep(2:11, 10)

# no grid again :(
rect(x1,y1,x2,y2, border="blue")

par <- op

#=====================================================================





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