Draw markers

    :param data: data access object
    :param marker: full filename of the marker image
    :param f_tooltip: function to generate a tooltip on mouseover
    :param marker_preferred_size: size in pixel for the marker images

def markers(data, marker, f_tooltip=None, marker_preferred_size=32):
    """
    Draw markers

    :param data: data access object
    :param marker: full filename of the marker image
    :param f_tooltip: function to generate a tooltip on mouseover
    :param marker_preferred_size: size in pixel for the marker images
    """
    from geoplotlib.layers import MarkersLayer
    _global_config.layers.append(MarkersLayer(data, marker, f_tooltip, marker_preferred_size))