Internal function primarily used from Plot.
objectNames(calledFrom = "Plot", argClass = ".spadesPlotObjects",
argName = "")character vector of length 1, indicating which function
call is desired. Defaults to Plot.
character vector of length 1, indicating which class is
being searched for among the arguments.
Defaults to .spadesPlotObjects.
character vector of length 1, or NULL, indicating
if the arguments to select have a name, no name (empty
string), or do not use name (NULL).
NULL. This function is invoked for its side effects.