The user can run this program either interactively or non-interactively. If the user simply types d.rhumbline on the command line and runs it without specifying parameter values, the mouse will be activated and the user will be asked to use the mouse to indicate the two endpoints of the rhumbline. The rhumbline is then drawn in the default color (black).
Alternately, the user can specify the starting and ending longitude/latitude coordinates of the rhumbline and (optionally) the color in which the rhumbline will be displayed; in this case, the program will run non-interactively.
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