Chart.js 円グラフのサンプル

<div style="width: 300px;">
  <canvas id="myChart"></canvas>
</div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.7.1/Chart.min.js"></script>

var ctx = document.getElementById('myChart');
var myChart = new Chart(ctx, {
  type: 'pie',
  data: {
    labels: ['赤党', '緑党', '青党'],
    datasets: [{
      label: '投票数',
      data: [10000, 5000, 7000],
      backgroundColor: [
        'rgba(255, 0, 0, 0.2)',
        'rgba(0, 255, 255, 0.2)',
        'rgba(0, 0, 255, 0.2)'
      ],
      borderColor: [
        'red',
        'green',
        'blue'
      ],
      borderWidth: 1
    }]
  },
  options: {
    tooltips: {
      callbacks: {
        label: function(item, data) {
          var dataset = data.datasets[item.datasetIndex];
          var total = dataset.data.reduce(function(prev, current) {
            return prev + current;
          });
          var current = dataset.data[item.index];
          var percentage = Math.floor(((current / total) * 100) + 0.5);
          return percentage + "%";
        }
      }
    }
  }
});