alias: atConnectionRatio

Move and auto-orient the subelement to the point at a given ratio of the connection total length. It accepts a number in the `[0,1]` range. It is valid only within the LinkView context.

    link.attr('rectSelector', { atConnectionRatio: .5, width: 10, height: 10, fill: 'red' });
    link.attr('rectSelector', { atConnectionRatioKeepGradient: .5, width: 10, height: 10, fill: 'red' });