_renderCrosshairItems() {
    const {values, itemsFormat} = this.props;
    const items = itemsFormat(values);
    if (!items) {
      return null;
    }
    return items
      .filter(i => i)
      .map(function renderValue(item, i) {
        return (
          <div className="rv-crosshair__item" key={`item${i}`}>
            <span className="rv-crosshair__item__title">{item.title}</span>
            {': '}
            <span className="rv-crosshair__item__value">{item.value}</span>
          </div>
        );
      });
  }
Render crosshair items (title + value for each series).
@returns {*} Array of React classes with the crosshair values.
@private