Create a dot density map

    :param data: data access object
    :param color: color
    :param point_size: point size
    :param f_tooltip: function to return a tooltip string for a point

def dot(data, color=None, point_size=2, f_tooltip=None):
    """Create a dot density map

    :param data: data access object
    :param color: color
    :param point_size: point size
    :param f_tooltip: function to return a tooltip string for a point
    """
    from geoplotlib.layers import DotDensityLayer
    _global_config.layers.append(DotDensityLayer(data, color=color, point_size=point_size, f_tooltip=f_tooltip))