_calculateForces(distance, dx, dy, forces, node) {
    if (distance > 0) {
      const degree = node.edges.length + 1;
      const gravityForce =
        this.options.centralGravity * degree * node.options.mass;
      forces[node.id].x = dx * gravityForce;
      forces[node.id].y = dy * gravityForce;
    }
  }
Calculate the forces based on the distance.
@param {number} distance
@param {number} dx
@param {number} dy
@param {Object<Node.id, Node>} forces
@param {Node} node
@private