Adaptation of grid that allows adding a grid to a map. This function will place the grid in the locations within the mapped area as delineated by the axes.
If you set the tick marks yourself, you can use add_abline to create a grid:
number of cells of the grid in x and y direction. When NULL, as per default, the grid aligns with the tick marks on the corresponding default axis (i.e., tickmarks as computed by axTicks). When NA, no grid lines are drawn in the corresponding direction
col
character or (integer) numeric; color of the grid lines
lty
character or (integer) numeric; line type of the grid lines
lwd
non-negative numeric giving line width of the grid lines