The whole sample frame is denoted by a matrix (nrow * ncol) in the
plane, and the sample points with equal intervals are drawn out according to
a random starting point. The points being sampled are marked by red circles.
sample.system(nrow = 10, ncol = 10, size = 15, p.col = c("blue", "red"), p.cex = c(1,
3))the desired number of rows of the sample frame.
the desired number of columns of the sample frame.
the sample size.
different colors / magnification rate to annotate the population and the sample
None (invisible NULL).
Examples at https://yihui.name/animation/example/sample-system/
sample, sample.simple,
sample.cluster, sample.ratio,
sample.strat