Chart.js/docs/samples/tooltip/content.md

# Custom Tooltip Content

This sample shows how to use the tooltip callbacks to add additional content to the tooltip.

```js chart-editor
// <block:footer:2>
const footer = (tooltipItems) => {
  let sum = 0;

  tooltipItems.forEach(function(tooltipItem) {
    sum += tooltipItem.parsed.y;
  });
  return 'Sum: ' + sum;
};

// </block:footer>

// <block:setup:1>
const DATA_COUNT = 7;
const NUMBER_CFG = {count: DATA_COUNT, min: -100, max: 100, decimals: 0};
const data = {
  labels: Utils.months({count: DATA_COUNT}),
  datasets: [
    {
      label: 'Dataset 1',
      data: Utils.numbers(NUMBER_CFG),
      fill: false,
      borderColor: Utils.CHART_COLORS.red,
      backgroundColor: Utils.transparentize(Utils.CHART_COLORS.red, 0.5),
    },
    {
      label: 'Dataset 2',
      data: Utils.numbers(NUMBER_CFG),
      fill: false,
      borderColor: Utils.CHART_COLORS.blue,
      backgroundColor: Utils.transparentize(Utils.CHART_COLORS.blue, 0.5),
    },
  ]
};
// </block:setup>

// <block:config:0>
const config = {
  type: 'line',
  data: data,
  options: {
    interaction: {
      intersect: false,
      mode: 'index',
    },
    plugins: {
      tooltip: {
        callbacks: {
          footer: footer,
        }
      }
    }
  }
};
// </block:config>

module.exports = {
  actions: [],
  config: config,
};
```

## Docs 
* [Data structures (`labels`)](../../general/data-structures.html)
* [Line](../../charts/line.html)
* [Tooltip](../../configuration/tooltip.html)
  * [Tooltip Callbacks](../../configuration/tooltip.html#tooltip-callbacks)