_calculateForces(distance, dx, dy, forces, node) {
    const gravityForce =
      distance === 0 ? 0 : this.options.centralGravity / distance;
    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, vis.Node>} forces
@param {Node} node
@private