code: function toggleClass(classNames) {
    const classes = classNames.split(' ');
  
    for(let i = 0; i < classes.length; i++) {
      const c = classes[i];
      this.classed(c, !this.classed(c));
    }
    return this;
  }
docs: jQuery equivalent: [$.toggleClass](http://api.jquery.com/toggleclass/)
  
  Adds or removes class from elements in the current selection. Returns current selection.
  
  @function toggleClass
  @example
  d3.selectAll('ul').toggleClass('active');
  d3.selectAll('ul').toggleClass('class-a class-b'); //toggle multiple classes
  
  @param {string}  className - The css class to toggle
  @returns {selection} - Returns current selection.