Legend Configuration

The chart legend displays data about the datasets that are 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.

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.

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: 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: {
            onClick: newLegendClickHandler
        }
    }
});

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.
        }
    }
});

Note that legendCallback is not called automatically and you must call generateLegend() yourself in code when creating a legend using this method.