Troubles with plot margins

I am trying to set the margins of a plot with mai and I get a strange output.
I write:
plot(rnorm(1000),type="l",mai=c(1,1,1,1))
and what I get is a plot with unchanged margins and a column of four 1 plotted on the top margin.

Marco Taboga (mtaboga at tiscalinet.it)
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