SUB show_graph(y,t,nshow)
    IF t = 0 then
       LET nshow = 10
       LET tmin = t
       LET tmax = 2               ! estimate of time to reach surface
       LET ymin = 0
       LET ymax = y
       CALL draw_axes(tmin,tmax,ymin,ymax)  ! see Appendix 3A
    END IF
    PLOT LINES: t,y;
END SUB