EXTERNAL
SUB draw_axes(xmin,xmax,ymin,ymax)
    LET ntick = 10                ! number of tick marks
    ! distance between tick marks on x-axis
    LET dx = (xmax - xmin)/ntick
    ! distance between tick marks on y-axis
    LET dy = (ymax - ymin)/ntick
    ! include margin
    SET WINDOW xmin - dx,xmax + dx,ymin - dy,ymax + dy
    LET x0 = max(0,xmin)
    LET y0 = max(0,ymin)
    IF ymin*ymax < 0 then
       LET y0 = 0
    ELSE
       LET y0 = ymin
    END IF
    PLOT LINES: xmin,y0;xmax,y0   ! horizontal axis
    PLOT LINES: x0,ymin;x0,ymax   ! vertical axis
    LET tx = 0.1*dy               ! size of tick mark
    LET ty = 0.1*dx
    FOR itick = 0 to ntick
        LET x = xmin + itick*dx
        PLOT LINES: x,y0 - tx; x,y0 + tx    ! draw ticks on x axis
        LET y = ymin + itick*dy
        PLOT LINES: x0 - ty,y; x0 + ty,y    ! draw ticks on y axis
    NEXT itick
END SUB

SUB compute_aspect_ratio(r,x,y)
    LET m = 0.1*r                 ! margin
    LET size = r + m
    ! px, py: # pixels in horizonal and vertical direction
    ASK PIXELS px,py
    IF px > py then
       LET aspect_ratio = px/py
       LET x = aspect_ratio*size
       LET y = size
    ELSE
       LET aspect_ratio = py/px
       LET x = size
       LET y = aspect_ratio*size
    END IF
END SUB