addBackgroundColor(fromValue, toValue, color) {
    this.rangeColors_.push(new GaugeColoredRange(fromValue, toValue, color));
    this.draw_();
  }
Set the background color for a range of values on the gauge.
@param {number} fromValue The lower (start) value of the colored range.
@param {number} toValue The higher (end) value of the colored range.
@param {string} color The color name to paint the range with. For example
    'red' or '#ffcc00'.