'set the legend title. Fontproperties can be optionally set
with *prop* parameter.'

def set_title(self, title, prop=None):

    self._legend_title_box._text.set_text(title)
    if (prop is not None):
        if isinstance(prop, dict):
            prop = FontProperties(**prop)
        self._legend_title_box._text.set_fontproperties(prop)
    if title:
        self._legend_title_box.set_visible(True)
    else:
        self._legend_title_box.set_visible(False)
    self.stale = True
