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
});