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legend.md

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title
Legend

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

Configuration options

Namespace: options.plugins.legend, the global options for the chart legend is defined in Chart.defaults.plugins.legend.

Name Type Default Description
display boolean true Is the legend shown?
position string 'top' Position of the legend. more...
align string 'center' Alignment of the legend. more...
maxHeight number Maximum height of the legend, in pixels
maxWidth number Maximum width of the legend, in pixels
fullSize boolean true Marks that this box should take the full width/height of the canvas (moving 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. Arguments: [event, legendItem, legend].
onHover function A callback that is called when a 'mousemove' event is registered on top of a label item. Arguments: [event, legendItem, legend].
onLeave function A callback that is called when a 'mousemove' event is registered outside of a previously hovered label item. Arguments: [event, legendItem, legend].
reverse boolean false Legend will show datasets in reverse order.
labels object See the Legend Label Configuration section below.
rtl boolean true for rendering the legends from right to left.
textDirection string canvas' default This will force the text direction 'rtl' or 'ltr' on the canvas for rendering the legend, regardless of the css specified on the canvas
title object See the Legend Title Configuration section below.

Position

Position of the legend. Options are:

  • 'top'
  • 'left'
  • 'bottom'
  • 'right'

Align

Alignment of the legend. Options are:

  • 'start'
  • 'center'
  • 'end'

Defaults to 'center' for unrecognized values.

Legend Label Configuration

Namespace: options.plugins.legend.labels

Name Type Default Description
boxWidth number 40 Width of coloured box.
boxHeight number font.size Height of the coloured box.
color Color Chart.defaults.color Color of label and the strikethrough.
font Font Chart.defaults.font See Fonts
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.
sort function null Sorts legend items. Receives 3 parameters, two Legend Items and the chart data.
pointStyle If specified, this style of point is used for the legend. Only used if usePointStyle is true.
usePointStyle boolean false Label style will match corresponding point style (size is based on the minimum value between boxWidth and font.size).

Legend Title Configuration

Namespace: options.plugins.legend.title

Name Type Default Description
color Color Chart.defaults.color Color of text.
display boolean false Is the legend title displayed.
font Font Chart.defaults.font See Fonts
padding number|object 0 Padding around the title. If specified as a number, it applies evenly to all sides.
text string The string title.

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,

    // Index of the associated dataset
    datasetIndex: number,

    // 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 | Image,

    // Rotation of the point in degrees (only used if usePointStyle is true)
    rotation: number
}

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: {
        plugins: {
            legend: {
                display: true,
                labels: {
                    color: '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, legend) {
    const index = legendItem.datasetIndex;
    const ci = legend.chart;
    if (ci.isDatasetVisible(index)) {
        ci.hide(index);
        legendItem.hidden = true;
    } else {
        ci.show(index);
        legendItem.hidden = false;
    }
}

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.plugins.legend.onClick;
var newLegendClickHandler = function (e, legendItem, legend) {
    var index = legendItem.datasetIndex;

    if (index > 1) {
        // Do the original logic
        defaultLegendClickHandler(e, legendItem);
    } else {
        let ci = legend.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: {
        plugins: {
            legend: {
                onClick: newLegendClickHandler
            }
        }
    }
});

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