Chart.js/test/fixtures/plugin.tooltip/opacity.js

var patternCanvas = document.createElement('canvas');
var patternContext = patternCanvas.getContext('2d');

patternCanvas.width = 6;
patternCanvas.height = 6;
patternContext.fillStyle = '#ff0000';
patternContext.fillRect(0, 0, 6, 6);
patternContext.fillStyle = '#ffff00';
patternContext.fillRect(0, 0, 4, 4);

var pattern = patternContext.createPattern(patternCanvas, 'repeat');

var gradient;

module.exports = {
  config: {
    type: 'line',
    data: {
      datasets: [{
        data: [8, 8, 8, 8, 8, 8, 7, 8, 8, 8, 8],
        pointBorderColor: '#ff0000',
        pointBackgroundColor: '#00ff00',
        showLine: false
      }, {
        label: '',
        data: [4, 4, 4, 4, 4, 5, 3, 4, 4, 4, 4],
        pointBorderColor: pattern,
        pointBackgroundColor: pattern,
        showLine: false
      }, {
        label: '',
        data: [0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0],
        showLine: false
      }],
      labels: ['', '', '', '', '', '', '', '', '', '', '']
    },
    options: {
      scales: {
        x: {display: false},
        y: {display: false}
      },
      elements: {
        line: {
          fill: false
        }
      },
      plugins: {
        legend: false,
        title: false,
        filler: false,
        tooltip: {
          mode: 'nearest',
          intersect: false,
          callbacks: {
            label: function() {
              return '\u200b';
            },
          }
        },
      },

      layout: {
        padding: 15
      }
    },
    plugins: [{
      beforeDatasetsUpdate: function(chart) {
        if (!gradient) {
          gradient = chart.ctx.createLinearGradient(0, 0, 512, 256);
          gradient.addColorStop(0, '#ff0000');
          gradient.addColorStop(1, '#0000ff');
        }
        chart.config.data.datasets[2].pointBorderColor = gradient;
        chart.config.data.datasets[2].pointBackgroundColor = gradient;

        return true;
      },
      afterDraw: function(chart) {
        var canvas = chart.canvas;
        var rect = canvas.getBoundingClientRect();
        var point, event;

        for (var i = 0; i < 3; ++i) {
          for (var j = 0; j < 11; ++j) {
            point = chart.getDatasetMeta(i).data[j];
            event = {
              type: 'mousemove',
              target: canvas,
              clientX: rect.left + point.x,
              clientY: rect.top + point.y
            };
            chart._handleEvent(event);
            chart.tooltip.handleEvent(event);
            chart.tooltip.opacity = j / 10;
            chart.tooltip.draw(chart.ctx);
          }
        }
      }
    }]
  },
  options: {
    canvas: {
      height: 256,
      width: 512
    }
  }
};