[R] arrow plots

[Gavin Simpson]

On Tue, 2009-12-08 at 13:42 -0500, Cable, Samuel B Civ USAF AFMC
AFRL/RVBXI wrote:
> Am doing some vector plots with the arrows() function.  Works well.  But
> what I need to do is supply an arrow for scaling for the reader.  I need
> to plot an arrow of some known magnitude somewhere on the page
> (preferably outside the bounds of the plot, so that it can be seen
> clearly) with some text underneath it that says, for instance, "10
> kg-m/sec".  Any ideas?  Thanks.

You can plot outside the plotting region using the 'xpd' plotting
parameter:

plot(1:10, 1:10) ## dummy plot
op <- par(xpd = TRUE) ## change clipping parameter & save defaults
arrows(9, 10.75, 10, 10.75, length = 0.1) ## draw reference arrow
text(9.5, 11, 
     labels = expression(10 ~ kg ~ m ~ sec^{"-1"})) ## add key
par(op) ## reset plotting parameter defaults

You'll probably need to tweak the expression to get the units you
wanted.

HTH

G
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