R help - Mar 2012 - Plot of function seems to cut off near edge of domain

Hello helpful R folks,
I am simply trying to graph a quarter circle centered at the origin in the
first quadrant.  When I set the xlim of the plot to the radius of the
circle, the plot appears correct.  However, I'd like to see a slight
extension of the axes beyond the domain of the function itself.  When I do
this, a portion of the plot seems to be missing by the edge of the domain. 
Here is the code for both of the plots:

dev.off() 
plot.new()
#Set up two-figure plot
par(mfrow=c(1,2),pty='s')
g<-function(x){sqrt(2500-x^2)}
#Figure 1, with xlim at the radius of the circle
plot(g,axes=F,xlim=c(0,50),ylim=c(0,50))
axis(1,pos=0)
axis(2,pos=0)
#Figure 2, with xlim beyond the radius of the circle
plot(g,axes=F,xlim=c(0,60),ylim=c(0,60))
axis(1,pos=0)
axis(2,pos=0)

Notice that the second graph doesn't appear to intersect the x-axis, while
the first one does.  Any ideas why that might be the case?  Here's an image
of what I see in case that's useful:

http://r.789695.n4.nabble.com/file/n4507954/Cut_off_Quarter_Circle.png 

Thanks in advance for the help!

-Chad Mills

--
View this message in context:
http://r.789695.n4.nabble.com/Plot-of-function-seems-to-cut-off-near-edge-of-domain-tp4507954p4507954.html
Sent from the R help mailing list archive at Nabble.com.

Dear Chad,

your problem is linked to (1) the function returning NaNs from x values 
greater than 50, and (2) the fact that the function is estimated on a 
predefined number of points.

Calling plot for a function object is basically a wrapper for curve(). Your 
function g() is evaluated on the whole xlim domain, which will return NaN 
values for x>50 (Try g(60) ). In addition, curve() splits the x interval
(here
from 0 to 60) into a predifined number of points (n=101 is the default, see 
help(curve)) at which the function is estimated. In your code, the function is 
estimated at values x <- seq(0, 60, length=101), and g(x) that are not NaN
are
plotted. The largest x value (from the sequence) that doesn't return a NaN
is
max(x[!is.nan(g(x))]), which is 49.8.

One way to solve it is to explicitly specify the domain used to estimate the 
function, by using the from and to arguments that are passed to curve():

#Figure 2, with xlim beyond the radius of the circle 
plot(g,axes=F,from=0, to =50, xlim=c(0, 60), ylim=c(0,60)) 
axis(1,pos=0) 
axis(2,pos=0)

HTH

Matthieu

Matthieu Dubois 
Post-doctoral researcher
Psychology Department
Universit? Libre de Bruxelles