[R] mapping groups of states without interior borders

[Simon, Heather]

I have been using the "map" function in R and I was wondering if there is a way to map multi-state regions just showing the outline of the region without interior state borders.  For instance, I am calculating aggregated statistics using 10 multi-state regions.  I would like to color-code individual states based on my dataset but then to create black outlines around each of these 10 regions so that the map shows the delineations of the various regions for which I am reporting aggregated statistics.

When I use:
map('state', region = "Idaho", "Washington", "Oregon", add = TRUE, color = "black")
I get a plot that includes interior boundaries between these 3 states instead of an exterior border around this region.

Thanks,
Heather

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