function getHighlightDigit(highlightKey) {
  var highlightDigit = _highlightKeyMap[highlightKey];

  if (highlightDigit == null && _highlightNextDigit <= 32) {
    highlightDigit = _highlightKeyMap[highlightKey] = _highlightNextDigit++;
  }

  return highlightDigit;
}
Support hightlight/downplay record on each elements.
For the case: hover highlight/downplay (legend, visualMap, ...) and
user triggerred hightlight/downplay should not conflict.
Only all of the highlightDigit cleared, return to normal.
@param {string} highlightKey
@return {number} highlightDigit