I know this has got to be simple, but I have a added  an arrow to a graph
with:

arrows(5,8,8, predict(lmfit,data.frame(x=8)), length=0.1)

but its in the wrong position, correcting it and running again adds an new
arrow (which is what you would expect) so how do I

a) edit the existing arrow, and
b) delete it all together

As so often seems to be the case, some of the simplist things seem also to
be the most difficult to find the answer to.

Many thanks,

Graham

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