Use legend
for a secondary legend. Increase the bottom mar
gin and add legend with negative inset
, i.e. move away from plot:
library(fields)
x<- 1:10
y<- 1:15
z<- outer( x,y,"+")
# plot with extra margin at bottom (7)
par(mar=c(7,4,4,2)+0.1)
image.plot(x,y,z,col=gray.colors(10), xlab='', ylab='')
# create points
xp = sample(1:10,size=5)
yp = sample(1:10,size=5)
points(xp,yp,pch=21,bg=1:2,cex=1:2)
# add legend (might have to change inset if you resize the plot)
legend('bottom', horiz=T, legend=paste('type', 1:2), pt.cex=1:2, pch=21, pt.bg=1:2, xpd=NA, inset=c(0,-1..))