If identify or digitize is TRUE, a data frame of class
pointPairs with in its rows the point pairs identified (pairs of
row numbers in the original data set); if identify is F, a plot of the
variogram cloud, which uses plot.gstatVariogram
If in addition to identify, keep is also TRUE, an object
of class variogramCloud is returned, having attached to it attributes
"sel" and "text", which will be used in subsequent calls to plot.variogramCloud
with identify set to FALSE, to plot the text previously identified.
If in addition to digitize, keep is also TRUE, an object of
class variogramCloud is returned, having attached to it attribute
"poly", which will be used in subsequent calls to plot.variogramCloud
with digitize set to FALSE, to plot the digitized line.
In both of the keep = TRUE cases, the attribute ppairs of
class pointPairs is present, containing the point pairs identified.