A Plot is a specialised form of FrameSet, in which the base Frame describes a " graphical"
coordinate system and is associated with a rectangular plotting area in the underlying graphics
system. This plotting area is where graphical output appears. It is defined when the Plot is
created.
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). 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).
If a null Object pointer (AST__NULL) is given, a default 2-dimensional Frame will be used to describe the physical coordinate system. Labels, etc. may then be attached to this by setting the appropriate Frame attributes (e.g. Label(axis)) for the Plot.
The first pair of values should give the coordinates of the bottom left corner of the plotting area and the second pair should give the coordinates of the top right corner. The coordinate on the horizontal axis should be given first in each pair. Note that the order in which these points are given is important because it defines up, down, left and right for subsequent graphical operations.
The base Frame of the returned Plot will be a new Frame which is created by this function to represent the coordinate system of the underlying graphics system (graphical coordinates). It is given a Frame index of 1 within the Plot. The choice of base Frame (Base attribute) should not, in general, be changed once a Plot has been created (although you could use this as a way of moving the plotting area around on the plotting surface).
If a Frame is supplied (via the FRAME pointer), then it becomes the current Frame of the new Plot and is given a Frame index of 2.
If a FrameSet is supplied (via the FRAME pointer), then all the Frames within this FrameSet become
part of the new Plot (where their Frame indices are increased by 1), with the FrameSet’ s current
Frame becoming the current Frame of the Plot.
If a null Object pointer (AST__NULL) is supplied (via the FRAME pointer), then the returned Plot will contain two Frames, both created by this function. The base Frame will describe graphics coordinates (as above) and the current Frame will be a basic Frame with no attributes set (this will therefore give default values for such things as the Plot Title and the Label on each axis). Physical coordinates will be mapped linearly on to graphical coordinates.
An error will result if the Frame supplied (or the base Frame if a FrameSet was supplied) is not 2-dimensional.
A null Object pointer (AST__NULL) will be returned if this function is invoked with STATUS set to an error value, or if it should fail for any reason.