add(groupName, style) {
    // Only push group name once to prevent duplicates which would consume more
    // RAM and also skew the distribution towards more often updated groups,
    // neither of which is desirable.
    if (!this._groups.has(groupName)) {
      this._groupNames.push(groupName);
    }
    this._groups.set(groupName, style);
    return style;
  }
Add custom group style.
@param {string} groupName - The name of the group, a new group will be
created if a group with the same name doesn't exist, otherwise the old
groups style will be overwritten.
@param {object} style - An object containing borderColor, backgroundColor,
etc.
@returns {object} The created group object.