[R] Plotting a loess line when the abcissa variable has numerous NA's

[David Parkhurst]

I have a situation in which I want to plot a variable Y against X, and 
then to add a loess line to that plot.  My X variable is 366 elements 
long, and about 1/3 are NA's, scattered through the list.  None of the 
corresponding Y's are NA's.  Everything I’ve tried so far, by mimicking 
examples from four different books, produces a messy scramble of lines 
when I try to plot the loess curve.  How can I deal with this situation?
Thanks for any help.