When points in physical coordinate space are transformed by a Plot into graphics coordinates for plotting, they may not always yield valid coordinates, irrespective of any clipping being applied (). To indicate this, the resulting coordinate values will be set to the value AST__BAD ().
There are a number of reasons why this may occur, but typically it will be because physical coordinates only map on to a subset of the graphics coordinate space. This situation is commonly encountered with all-sky projections where, typically, the celestial sphere appears, when plotted, as a distorted shape (e.g. an ellipse) which does not entirely fill the graphics space. In some cases, there may even be multiple regions of valid and invalid physical coordinates.
When plotting is performed via a Plot, graphical output will
only appear in the regions of valid physical coordinates. Nothing will
appear where invalid coordinates occur. Such output is effectively
clipped. If you wish to plot in these areas, you must change
coordinate system and use, say, graphical coordinates to address the
plotting surface directly.
AST A Library for Handling World Coordinate Systems in Astronomy