PROGRAM plot_function             ! plot analytical solution
CALL initial(t,tmax,r,Ts,T0)
CALL set_up_window(t,tmax,Ts,T0)
CALL show_plot(t,tmax,r,Ts,T0)
END

SUB initial(t,tmax,r,Ts,T0)
    LET t = 0
    LET T0 = 83                   ! initial coffee temperature (C)
    LET Ts = 22                   ! room temperature (C)
    INPUT prompt "cooling constant r = ": r      ! cooling constant
    INPUT prompt "duration = ": tmax   ! time (minutes)
    CLEAR
END SUB

SUB set_up_window(xmin,xmax,ymin,ymax)
    LET mx = 0.01*(xmax - xmin)   ! margin
    LET my = 0.01*(ymax - ymin)
    SET WINDOW xmin - mx,xmax + mx,ymin - my,ymax + my
    ! default background color black on all computers except Macintosh
    PLOT xmin,ymin; xmax,ymin     ! abbreviation for PLOT LINES:
    PLOT xmin,ymin; xmin,ymax
END SUB

SUB show_plot(t,tmax,r,Ts,T0)
    DECLARE DEF f                 ! declare use of external function
    SET COLOR "red"
    DO while t <= tmax
       PLOT t,f(t,r,Ts,T0);       ! abbreviation for PLOT LINES
       LET t = t + 0.1
    LOOP
END SUB

DEF f(t,r,Ts,T0)
    LET f = Ts - (Ts - T0)*exp(-r*t)
END DEF