renderScaleHeading(max) {
    let keys = this.scale.map(ele => ele.name);
    if (typeof this.options.renderScaleHeading === 'function') {
      keys = this.options.renderScaleHeading.call(this, max);
    }
    let diff = this.scaleLength - keys.join('').length;
    let spacing = Math.round(diff / (keys.length - 1));
    let names = keys.map(key => this.styles.strong(key));
    let headings = names.join(' '.repeat(spacing));
    let padding = ' '.repeat(this.widths[0]);
    return this.margin[3] + padding + this.margin[1] + headings;
  }
Render the heading row for the scale.
@return {String}