When a Plot is created, it is initialised by providing a FrameSet whose base Frame (as specified by its Base attribute) is mapped linearly or logarithmically (as specified by the LogPlot attribues) on to a plotting area. This is a rectangular region in the graphical coordinate space of the underlying graphics system and becomes the new base Frame of the Plot. In effect, the Plot becomes attached to the plotting surface, in rather the same way that a basic FrameSet might be attached to (say) an image.
The current Frame of the Plot (derived from the current Frame of the FrameSet supplied) is used to represent a physical coordinate system. This is the system in which plotting operations are performed by your program. Every plotting operation is then transformed through the Mapping which inter-relates the Plot's current and base Frames in order to appear on the plotting surface.
An example may help here. Suppose we start with a FrameSet whose base Frame describes the pixel coordinates of an image and whose current Frame describes a celestial (equatorial) coordinate system. Let us assume that these two Frames are inter-related by a Mapping within the FrameSet which represents a particular sky projection.
When a Plot is created from this FrameSet, we specify how the pixel coordinates (the base Frame) maps on to the plotting surface. This simply corresponds to telling the Plot where we have previously plotted the image data. If we now use the Plot to plot a line with latitude zero in our physical coordinate system, as given by the current Frame, this line would appear as a curve (the equator) on the plotting surface, correctly registered with the image.
There are a number of plotting functions provided, which all work in a similar way. Plotting operations are transformed through the Mapping which the Plot represents before they appear on the plotting surface.31 It is possible to draw symbols, lines, axes, entire grids and more in this way.
AST A Library for Handling World Coordinate Systems in Astronomy