[R] Plot of function seems to cut off near edge of domain

[chad.mills]

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

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