Export a trellis object as a pdf, jpg or similar

Hello everyone!

I am drawing maps using spplot function, from sp package, and I am
experiencing problems in visualizing my maps because apparently the volume
of data is big. To avoid waiting I've thought in creating an object and
then export it in another format such as pdf, jpg, png...

I've tried pdf(), png(), but what it is recorded is an empty file. What
else can I try?

Or if not, is there any way of diminishing the time R takes to show the map
I'm drawing?

I let here my trellis object:

world_threat <- spplot(World_map, "total_amenazadas", at=world_pretty$brks,
col.regions=pal, sp.layout=spl, col = "grey30")

Thank you very much,

Ester

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