The 'connectionRatio' link anchor function places the anchor of the link at a point at a specified ratio of the reference link's length (from reference link's source). It accepts one argument, which can be passed within the linkAnchor.args property:

ratio number The length ratio of the target anchor. Default is 0.5, meaning the midpoint of the reference link.

Example:

link.source(link2, {
    linkAnchor: {
        name: 'connectionRatio',
        args: {
            ratio: 0.25
        }
    }
});