Rearrange x-y Data for Fast Plotting
DESCRIPTION:
Returns a list with the data re-ordered in such a way that pen plotters will plot faster.
USAGE: 
xysort(x, y, order=F)
REQUIRED ARGUMENTS:
x,y:
coordinates of points. The coordinates can be given by two arguments that are vectors or by a single argument x which is a univariate time series, a complex vector, a matrix with 2 columns, or a list containing components named x and y.
Missing values (NA) are allowed.
OPTIONAL ARGUMENTS:
order:
if TRUE, ordering vector is returned.
VALUE:
list with components named x and y, giving a rearranged version of the input data points, plus an optional order component.
x,y:
rearranged values of original x and y data. Points with NAs in either the original x or y coordinates are omitted.
order:
a permutation vector is returned. This permutation can be used to rearrange the original x, y, or any other associated data.
DETAILS:
Adjacent values in the returned x and y data are likely to be near one another in x-y space. The idea is that a mechanical plotter will be able to visit all of the points much faster when they are presented in the sorted order.

A greedy algorithm is used, finding the nearest neighbor to the first point, then the nearest neighbor to that point, etc. The algorithm has complexity O(length(x)^2).

REFERENCES:
Jon Louis Bentley (1984). A Case Study in Applied Algorithm Design. IEEE Computer, 75-88.

Jon Louis Bentley (1982). Writing Efficient Programs. Prentice-Hall, Englewood Cliffs, NJ.

SEE ALSO:
sort , order .
EXAMPLES: 
plot( xysort(x,y) )


xyo <- xysort(x, y, order=T)
plot(xyo, type="n")
text(xyo, label[xyo$order])