the default behoviour to plot towns with fixed size
a new argument towns, can be either a boolean or a character indicating the variable to map with the size of the points (currently only "pop")
a new size argument, length one for a fixed size and lenght two to define the range of the sizes of the points
a new shape argument, length one to define the shape of the points or length two to map the shape with the capital
new oceans and background argument to set the colour of the oceans and of the background