A string which uniquely identifies a plot plotscaper scene or schema.
idAn object of class NULL of length 0.
id is a string that uniquely identifies a plot within a
plotscaper scene or schema. It can match a plot
based on its position (e.g. "plot1", "plot2", ...),
in the order the plots were added, left-to-right top-to-bottom,
or it can match plot based on type (e.g. "scatter1" or "barplot3"),
again, in order of addition.
If the plot is matched based on type, the morphemes "plot" and "gram" are ignored, such that e.g. "scatterplot1" is the same as "scatter1" and "histogram2d4" is the same as "histo2d4".
The string can also be shortened, e.g. "p1" for "plot1", "s2" for "scatter2", or "hh3" for "histo2d3".