'Return a format string formatting the *x*, *y* coord'

def format_coord(self, x, y):

    if (x is None):
        xs = u'???'
    else:
        xs = self.format_xdata(x)
    if (y is None):
        ys = u'???'
    else:
        ys = self.format_ydata(y)
    return (u'x=%s    y=%s' % (xs, ys))
