# Legend Configuration

The chart legend displays data about the datasets that area appearing on the chart.

## Configuration options

The legend configuration is passed into the `options.legend` namespace. The global options for the chart legend is defined in `Chart.defaults.global.legend`.

Name | Type | Default | Description
--- | --- | --- | ---
`display` | `Boolean` | `true` | is the legend shown
`position` | `String` | `'top'` | Position of the legend. more...
`fullWidth` | `Boolean` | `true` | Marks that this box should take the full width of the canvas (pushing down other boxes). This is unlikely to need to be changed in day-to-day use.
`onClick` | `Function` |  | A callback that is called when a click event is registered on a label item
`onHover` | `Function` |  | A callback that is called when a 'mousemove' event is registered on top of a label item
`reverse` | `Boolean` | `false` | Legend will show datasets in reverse order.
`labels` | `Object` |  | See the Legend Label Configuration section below.

## Position

Position of the legend. Options are:

- `'top'`
- `'left'`
- `'bottom'`
- `'right'`

## Legend Label Configuration

The legend label configuration is nested below the legend configuration using the `labels` key.

Name | Type | Default | Description
--- | --- | --- | ---
`boxWidth` | `Number` | `40` | width of coloured box
`fontSize` | `Number` | `12` | font size of text
`fontStyle` | `String` | `'normal'` | font style of text
`fontColor` | `Color` | `'#666'` | Color of text
`fontFamily` | `String` | `"'Helvetica Neue', 'Helvetica', 'Arial', sans-serif"` | Font family of legend text.
`padding` | `Number` | `10` | Padding between rows of colored boxes.
`generateLabels` | `Function` |  | Generates legend items for each thing in the legend. Default implementation returns the text + styling for the color box. See Legend Item for details.
`filter` | `Function` | `null` | Filters legend items out of the legend. Receives 2 parameters, a Legend Item and the chart data.
`usePointStyle` | `Boolean` | `false` | Label style will match corresponding point style (size is based on fontSize, boxWidth is not used in this case).

## Legend Item Interface

Items passed to the legend `onClick` function are the ones returned from `labels.generateLabels`. These items must implement the following interface.

    {
        // Label that will be displayed
        text: String,

        // Fill style of the legend box
        fillStyle: Color,

        // If true, this item represents a hidden dataset. Label will be rendered with a strike-through effect
        hidden: Boolean,

        // For box border. See https://developer.mozilla.org/en/docs/Web/API/CanvasRenderingContext2D/lineCap
        lineCap: String,

        // For box border. See https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/setLineDash
        lineDash: Array[Number],

        // For box border. See https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/lineDashOffset
        lineDashOffset: Number,

        // For box border. See https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/lineJoin
        lineJoin: String,

        // Width of box border
        lineWidth: Number,

        // Stroke style of the legend box
        strokeStyle: Color

        // Point style of the legend box (only used if usePointStyle is true)
        pointStyle: String
    }

## Example

The following example will create a chart with the legend enabled and turn all of the text red in color.

    var chart = new Chart(ctx, {
        type: 'bar',
        data: data,
        options: {
            legend: {
                display: true,
                labels: {
                    fontColor: 'rgb(255, 99, 132)'
                }
            }
    }
    });

## Custom On Click Actions

It can be common to want to trigger different behaviour when clicking an item in the legend. This can be easily achieved using a callback in the config object.

The default legend click handler is:

    function(e, legendItem) {
        var index = legendItem.datasetIndex;
        var ci = this.chart;
        var meta = ci.getDatasetMeta(index);

        // See controller.isDatasetVisible comment
        meta.hidden = meta.hidden === null? !ci.data.datasets[index].hidden : null;

        // We hid a dataset ... rerender the chart
        ci.update();
    }

Lets say we wanted instead to link the display of the first two datasets. We could change the click handler accordingly.

    var defaultLegendClickHandler = Chart.defaults.global.legend.onClick;
    var newLegendClickHandler = function (e, legendItem) {
        var index = legendItem.datasetIndex;

        if (index > 1) {
            // Do the original logic
            defaultLegendClickHandler(e, legendItem);
        } else {
            let ci = this.chart;
            [ci.getDatasetMeta(0),
             ci.getDatasetMeta(1)].forEach(function(meta) {
                meta.hidden = meta.hidden === null? !ci.data.datasets[index].hidden : null;
            });
            ci.update();
        }
    };

    var chart = new Chart(ctx, {
        type: 'line',
        data: data,
        options: {
            legend: {

            }
        }
    });

Now when you click the legend in this chart, the visibility of the first two datasets will be linked together.

## HTML Legends

Sometimes you need a very complex legend. In these cases, it makes sense to generate an HTML legend. Charts provide a `generateLegend()` method on their prototype that returns an HTML string for the legend.

To configure how this legend is generated, you can change the `legendCallback` config property.

    var chart = new Chart(ctx, {
        type: 'line',
        data: data,
        options: {
            legendCallback: function(chart) {
                // Return the HTML string here.
            }
        }
    });

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