Valid for `<path>` subelements. It draws an SVG element at the beginning of a path. The element is automatically rotated based on the path direction. It's defined as an object with `type` property and any other visual attributes. The valid values for `type` are `'path'`, `'circle'`, `'ellipse'`, `'rect'`, `'polyline'` and `'polygon'`.

    link.attr('.connection/sourceMarker', {
      type: 'circle', // SVG Circle
      fill: '#666',
      stroke: '#333',
      r: 5, // radius of the circle
      cx: 5 // move the centre of the circle 5 pixels from the end of the path
    });