有关cnFillOn开了无法填充的问题

一大碗年糕

格点数据在画等值线，想填充等值线之间的区域但是发现cnFillOn设置为True后发现无论怎么设置都无法填充，但是等值线是可以画出来的，研究了一下发现cnFillMode有三种选项，默认时是AreaFill,还有RasterFill栅格填充和CellFill，发现换为这两种模式是可以填充的，请教大家以下到底是什么原因呢？等值线部分代码：

res@cnLinesOn = True
res@cnFillOn = True
res@cnFillMode = "RasterFill"
res@cnRasterSmoothingOn = True
res@cnLevelSelectionMode = "ExplicitLevels"
res@cnLevels = (/-1,0.3044,1.0/)
res@cnLineDashPattern = 2
res@cnLineThicknessF = 3
res@cnFillPalette = "gsltod"
res@cnFillColors = (/-1,-1,8,-1/)
res@cnFillOpacityF = 0.8
res@cnLineLabelsOn = False
;res@gsnContourZeroLineThicknessF = 0
res@cnInfoLabelOn = False
res@lbLabelBarOn = False


res@tiMainString = "~F9~ Nanjing r-test of January"
res@tiMainOffsetYF = -0.035
res@gsnLeftString = ""

res@pmTickMarkDisplayMode = "Always"
res@lbBoxLinesOn = False
plot = gsn_csm_contour(wks, data, res)