图例条目没有标准的工具提示，但是您可以添加自己的...

no standard tooltip for legend entries, but you can add your own...

参见以下工作片段，

在这里，我使用图表事件 onmouseover 和 onmouseout ，

知道何时传说已悬停

here, i use chart events onmouseover and onmouseout,
to know when the legend has been "hovered"

事件发生时，如果 row 事件的属性为 null ，

，则说明将悬停在图例

when the events fire, if the row property of the event is null,
then the legend is being hovered

我还使用 getChartLayoutInterface 将工具提示放置在图例附近

i also use getChartLayoutInterface to position the tooltip near the legend

google.charts.load('current', {
  callback: drawBasic,
  packages: ['corechart']
});

function drawBasic() {
  var data = google.visualization.arrayToDataTable([
    ['City', '2010 Population',],
    ['New York City, NY', 8175000],
    ['Los Angeles, CA', 3792000],
    ['Chicago, IL', 2695000],
    ['Houston, TX', 2099000],
    ['Philadelphia, PA', 1526000]
  ]);

  var options = {
    title: 'Population of Largest U.S. Cities',
    chartArea: {width: '50%'},
    hAxis: {
      title: 'Total Population',
      minValue: 0
    },
    vAxis: {
      title: 'City'
    }
  };

  var chart = new google.visualization.BarChart(document.getElementById('chart_div'));
  var legendTooltip = document.getElementById('legend-tooltip');

  // set legend tooltip position
  google.visualization.events.addListener(chart, 'ready', function (gglEvent) {
    var chartLayout = chart.getChartLayoutInterface();
    var legendBounds = chartLayout.getBoundingBox('legend');
    legendTooltip.style.top = (legendBounds.top + (legendBounds.height * 2)) + 'px';
    legendTooltip.style.left = legendBounds.left + 'px';
  });

  // show legend tooltip
  google.visualization.events.addListener(chart, 'onmouseover', function (gglEvent) {
    if (gglEvent.row === null) {
      $('#series-name').html(data.getColumnLabel(gglEvent.column));
      $(legendTooltip).removeClass('hidden');
    }
  });

  // hide legend tooltip
  google.visualization.events.addListener(chart, 'onmouseout', function (gglEvent) {
    if (gglEvent.row === null) {
      $(legendTooltip).addClass('hidden');
    }
  });

  chart.draw(data, options);
}

.hidden {
  display: none;
  visibility: hidden;
}

.ggl-tooltip {
  background-color: #ffffff;
  border: 1px solid #E0E0E0;
  display: inline-block;
  font-size: 10pt;
  padding: 8px 8px 8px 8px;
  position: absolute;
}

.ggl-tooltip div {
  margin-top: 4px;
}

.ggl-tooltip span {
  font-weight: bold;
}

<script src="https://www.gstatic.com/charts/loader.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<div id="chart_div"></div>
<div id="legend-tooltip" class="ggl-tooltip hidden">
  <div><span>Series Info</span></div>
  <div id="series-name"></div>
</div>