text: /**
   * Get a d3 selection of the workspace <div>. Create if not exist.
   * @version 1.0
   * @date 20200508
   */
  function get_workspace() {
      const master = d3.select(".master-container");
      if (master.select("#workspace").empty()) {
          master.append("canvas")
              .attr("id", "workspace")
              .attr("width", "600")
              .attr("height", "800");
      }
      return master.select("#workspace");
  }
  