DataViewx mouse actions
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       *  MOUSE on plot:

           * button 1: zoom-in

               Pressing down button 1 starts a zoom-in operation.
               Holding button 1 down and moving to new location displays
               a zoom-in box with endpoints at original location and
               current location.  Releasing button 1 changes axes limits
               of graph to those of the zoom-in box.

           * button 2: restore

               Pressing down button 2 returns axes limits to the original ones.

           * button 3: line parameter report

               Pressing down button 3 starts a line-segment operation
               for reporting slope and intercepts.  Holding down, moving,
               then releasing define a line-segment.  After release,
               a report of the line-parameters of the defined line segment:
               slope, y-intercept, x-intercept.
               For a single press and release, only x and y are reported.

           * shift-button 3: curve label operation

               Pressing down button 3 with shift held down starts a
               curve-label operation.  Holding down, moving, then releasing
               defines a line-segment.  After release, prompt for text for
               the label, and whether or not to add an arrow.

           * shift-button 1: delete curve label operation

               Pressing down button 3 with shift held down starts a delete
               curve-label operation.  Holding down, moving, then releasing
               defines a delete box.  After release, any markers within the
               delete box are removed.

           * control-button 3: data removal operation

               Pressing down button 3 with control held down starts a
               data removal operation.  Holding down, moving, then releasing
               defines a delete box (red).  After release, any data points
               (XCOL, YCOL) are deleted from the working data set.
        
       *  MOUSE on note-book tab:

           * button 1: select page

               Button 1 selects notebook page connected to the tab.