The camera is in charge of projecting the graph spaces in graph units (GU) into user spaces (often in pixels). It defines the transformation (an affine matrix) to passe from the first to the second. It also contains the graph metrics, a set of values that give the overall dimensions of the graph in graph units, as well as the view port, the area on the screen (or any rendering surface) that will receive the results in pixels (or rendering units).
The camera defines a centre at which it always points. It can zoom on the graph, pan in any direction and rotate along two axes.
Knowing the transformation also allows to provide services like "what element is not invisible ?" (not in the camera view) or "on what element is the mouse cursor actually ?".
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