Works like cut to select a range of data. This range is fitted to a straight line (y=a+bx) each time the limits are changed (by using the left/right buttons or b/e, as in Cut). The slope (b) and the intercept (-b/a) are written on the screen.
Type q to Quit doing nothing.
Use v to quit dividing by the straight line.
Type u to quit replacing the data with the data minus the straight baseline.