Add geographical axes to a map

map.axes2(sides = c(1, 2), cex.axis = 0.75, line = -0.4)

Arguments

sides: Side of the axis, 1 and 2 by default. See graphics::axis.
cex.axis: Character expansion factor for axis labels.
line: the number of lines into the margin at which the axis line will be drawn, if not NA.