The AST_BORDER routine is provided to draw a (line) border around
your graphical output. With most graphics systems, this would simply
be a rectangular box around the plotting area. With a Plot, however,
this boundary follows the edge of each region containing valid,
unclipped physical coordinates ().
This means, for example, that if you were plotting an all-sky projection, this boundary would outline the perimeter of the celestial sphere when projected on to your plotting surface. Of course, if there is no clipping and all physical coordinates are valid, then you will get the traditional rectangular box. AST_BORDER requires only a pointer to the Plot and the usual STATUS argument:
LOGICAL HOLES ... HOLES = AST_BORDER( PLOT, STATUS )
It returns a logical value to indicate if any invalid or clipped
physical coordinates were found within the plotting area. If they
were, it will draw around the valid unclipped regions and return
.TRUE.. Otherwise, it will draw a simple rectangular border and return
.FALSE..
AST A Library for Handling World Coordinate Systems in Astronomy