_createSeperator(index = 1) {
    this.manipulationDOM["seperatorLineDiv" + index] =
      document.createElement("div");
    this.manipulationDOM["seperatorLineDiv" + index].className =
      "vis-separator-line";
    this.manipulationDiv.appendChild(
      this.manipulationDOM["seperatorLineDiv" + index]
    );
  }
create a seperator line. the index is to differentiate in the manipulation dom
@param {number} [index]
@private