'on button press event'

def _press(self, event):

    self.rect.set_visible(self.visible)
    if self.span_stays:
        self.stay_rect.set_visible(False)
        if self.useblit:
            self.canvas.draw()
    (xdata, ydata) = self._get_data(event)
    if (self.direction == u'horizontal'):
        self.pressv = xdata
    else:
        self.pressv = ydata
    return False
