White overlaps country borders

Hi @abatalla and @vagudets (fyi @vtorralba @nmilders )

I'm working on the dev-test_bigpredidata branch making plots of forecasts like the one attached. I'm masking with white those grids where the probability of the most likely tercile is below 40%, where RPSS <0 and where there is a NaN.

One of those 3 conditions (not all of them) is being applied after painting the country borders, making them dissappear underneath the white square, for example the border of Argelia are mostly underneath white.

I suspect it's the col.NA who's being painted on top of the borders because it's precipitation and in super dry regions there are normally many nans after bias correcting. Is there any way to paint the country borders at the end to avoid this issue? or ask for zorder = 0 (not sure if there's an option to do this in R).

Many thanks for your help,

Paloma