SYNOPSIS:
    MARKGG_PLANEADD axis planenum [planefact] [bgfact] [offset]

DESCRIPTION:
    Add a given plane to all planes in a volume.
    The name is somewhat misleading, as its main use is to _subtract_
    a given plane to allow for digital subtraction angiography.
    If planefact and/or bgfact are not given, they default to -1.0
    and 1.0, thus yielding subtraction of the plane as standard
    behaviour.
    If offset is not given, it defaults to 0, if both factors are
    positive and 1000 otherwise.
