XYplotm line parameters
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       * Finding slope, y-intercept and other line-parameters from
         line drawn on plot.

            1. press down button-3 at the first corner of a line-segment.

            2. hold down the button and move to the opposite corner of the 
               line-segment.   

            3. release button-3.
               A line-parameter report is printed out to standard output.
               The report gives the line-parameters of the
               defined line segment: slope, y-intercept, x-intercept and
               other parameters.
               For a single press and release, only x and y are reported.