折线图上同时画两个物理量

lc2016

begin
y1=(/1,1,1,2,1,28,42,9,49,83,8,2.0,2.0,3.0,8.0/)
y2=(/232,250,260,270,260,220,210,205,200,196,196,196,210,210,214/)
time=(/0,1,2,3,4,5,6,7,8,9,10,11,12,13,14/)
wks=gsn_open_wks("png","xy")

resL=True
resL@xyLineThicknesses=5
resL@tiYAxisString=""
resL@trYMaxF =90
resL@trYMinF =0
resL@tiMainString = ""
resL@xyLineColors = "blue"

resR=True
resR@tiYAxisString=""
resR@xyLineThicknesses=5

resR@trYMaxF           =280
  resR@trYMinF           =190
  resR@xyLineColors      = "red"
resR@xyExplicitLegendLabels = "a,b"
plot=gsn_csm_xy2(wks,time,y1,y2,resL,resR)
end
如何添加两个物理量的标签呢或者说图例？