The index of the Frame within the Plot to which the clipping
limits supplied in "lbnd" and "ubnd" (below) refer. Clipping
may be applied to any of the coordinate systems associated
with a Plot (as defined by the Frames it contains), so this
index may take any value from 1 to the number of Frames in
the Plot (Nframe attribute). In addition, the values
AST__BASE and AST__CURRENT may be used to specify the base
and current Frames respectively.
For example, a value of AST__CURRENT causes clipping to be
performed in physical coordinates, while a value of AST__BASE
would clip in graphical coordinates. Clipping may also be
removed completely by giving a value of AST__NOFRAME. In this
case any clipping bounds supplied (below) are ignored.