The current Frame of a Plot describes a "physical" coordinate system, which is the coordinate system in which plotting operations are specified. The results of each plotting operation are automatically transformed into graphical coordinates so as to appear in the plotting area (subject to any clipping which may be in effect).
Because the Mapping between physical and graphical coordinates may often be non-linear, or even discontinuous, most plotting does not result in simple straight lines. The basic plotting element is therefore not a straight line, but a geodesic curve (see AST_CURVE, AST_GENCURVE and AST_POLYCURVE). A Plot also provides facilities for drawing markers or symbols (AST_MARK), text (AST_TEXT) and grid lines (AST_GRIDLINE). It is also possible to draw curvilinear axes with optional coordinate grids (AST_GRID). A range of Plot attributes is available to allow precise control over the appearance of graphical output produced by these routines.
You may select different physical coordinate systems in which to plot (including the native graphical coordinate system itself) by selecting different Frames as the current Frame of a Plot, using its Current attribute. You may also set up clipping (see AST_CLIP) to limit the extent of any plotting you perform, and this may be done in any of the coordinate systems associated with the Plot, not necessarily the one you are plotting in.
Like any FrameSet, a Plot may also be used as a Frame. In this case, it behaves like its current Frame, which describes the physical coordinate system.
When used as a Mapping, a Plot describes the inter-relation between graphical coordinates (its base Frame) and physical coordinates (its current Frame). It differs from a normal FrameSet, however, in that an attempt to transform points which lie in clipped areas of the Plot will result in bad coordinate values (AST__BAD).
AST A Library for Handling World Coordinate Systems in Astronomy