 Usage:  (xyline)=steps4plot(xy, xymin, xmax)  
 
 Input:

  xy                     n x 2 matrix, coordinates of the jump points sorted 
                         by the first column 
                         
  xymin                  1 x 2 matrix, coordinates of the starting point. 
                         Default is the first jump point. Its x coordinate is 
                         set to that of the first jump if specified as larger. 
                         
  xmax                   scalar, x coordinate of the ending point Default is 
                         by adding 1% of the x range to the last jump point. 
                         Its x coordinate is set to that of the last first 
                         jump if specified as smaller. Y coordinate of the 
                         ending point is always that of the last jump point. 
                         
 Output:

  xyline                 4*(n+1) x 2 matrix, the points to be connected into 
                         step function. 
                         
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(C) MD*TECH Method and Data Technologies, 17.8.2000
