Like many graphics systems, a Plot allows you to clip the graphics you produce. This means that plotting is restricted to certain regions of the plotting surface so that anything drawn outside these regions will not appear. All Plots automatically clip at the edges of the plotting area specified when the Plot is created. This means that graphics are ultimately restricted to the rectangular region of plotting space to which you have attached the Plot.
In addition to this, you may also specify lower and upper limits on each axis at which clipping should occur. This permits you to further restrict the plotting region. Moreover, you may attach these clipping limits to any of the Frames in the Plot. This allows you to place restrictions on where plotting will take place in either the physical coordinate system, the graphical coordinate system, or in any other coordinate system which is described by a Frame within the Plot.
For example, you could plot using equatorial coordinates and set up clipping limits in galactic coordinates. In general, you could set up arbitrary clipping regions by adding a new Frame to a Plot (in which clipping will be performed) and inter-relating this to the other Frames in a suitable way.
Clipping limits are defined using the AST_CLIP routine, as follows:
INTEGER IFRAME, NAXES DOUBLE PRECISION LBND( NAXES ), UBND( NAXES ) ... CALL AST_CLIP( PLOT, IFRAME, LBND, UBND, STATUS )
Here, the IFRAME value gives the index of the Frame within the Plot to which clipping is to be applied, while LBND and UBND give the limits on each axis of the selected Frame (NAXES is the number of axes in this Frame).
You can remove clipping by giving a value of AST__NOFRAME for IFRAME.
AST A Library for Handling World Coordinate Systems in Astronomy