plt = plot(X, Y, /buffer)
line = polyline([x1, x2], [y1, y2], target = plt, /data)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; x [0, 2π] で
;; y = abs(cos(x)) をプロットし、
;; 図の全体に バツ印を書いてみる。
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
pro sample_polyline
saveDirName = '~/Downloads/'
saveFileName = 'figure_sample_polyline.png'
n = 100d
x = dindgen(n, start = 0, increment = 2d*!pi/n)
y = abs(cos(x))
plt = plot(x, y,$
    ; x 
    xtitle = 'x [rad]',$
    xrange = [0, 2d*!pi],$
    xtickvalues = [0, !pi, 2d*!pi],$
    xtickname = ['0', '$\pi$', '2$\pi$'],$
    xminor = 1,$
    ; y
    ytitle = '|cos(x)|',$
    yrange = [0, 1],$
    ytickvalues = [0, 1],$
    ytickname = ytickvalues,$
    yminor = 4,$
    ; title
    title = '',$
    ; linestyle
    linestyle = 0,$ ; solid line
    thick = 2,$
    color = 'black',$
    ; symbol
    symbol = 'o',$
    sym_size = 0,$
    sym_color = 'black',$
    sym_filled = 1,$
    ; font
    font_size = 14,$
    font_name = 'Helvetica',$
    ; option
    /buffer)
line = polyline([0, 2d*!pi], [1, 0],$
    target = plt,$
    linestyle = 2,$ ; dashed line
    color = 'black',$
    /data)
line = polyline([0, 2d*!pi], [0, 1],$
    target = plt,$
    linestyle = 2,$ ; dashed line
    color = 'black',$
    /data)
plt.save, saveDirName + saveFileName
end