[Chartjs]-Make Chart.js Radar labels clickable

I came up with a solution for this for version 2.8.0 by copying the label position calculations from the RadialLinear scale into an event handler.

document.getElementById("myChart").onclick = function (e) {
    var helpers = Chart.helpers;
    var scale = myRadarChart.scale;
    var opts = scale.options;
    var tickOpts = opts.ticks;

    // Position of click relative to canvas.
    var mouseX = e.offsetX;
    var mouseY = e.offsetY;

    var labelPadding = 5; // number pixels to expand label bounding box by

    // get the label render position
    // calcs taken from drawPointLabels() in scale.radialLinear.js
    var tickBackdropHeight = (tickOpts.display && opts.display) ?
        helpers.valueOrDefault(tickOpts.fontSize, Chart.defaults.global.defaultFontSize)
        + 5: 0;
    var outerDistance = scale.getDistanceFromCenterForValue(opts.ticks.reverse ? scale.min : scale.max);
    for (var i = 0; i < scale.pointLabels.length; i++) {
        // Extra spacing for top value due to axis labels
        var extra = (i === 0 ? tickBackdropHeight / 2 : 0);
        var pointLabelPosition = scale.getPointPosition(i, outerDistance + extra + 5);

        // get label size info.
        // TODO fix width=0 calc in Brave?
        // https://github.com/brave/brave-browser/issues/1738
        var plSize = scale._pointLabelSizes[i];

        // get label textAlign info
        var angleRadians = scale.getIndexAngle(i);
        var angle = helpers.toDegrees(angleRadians);
        var textAlign = 'right';
        if (angle == 0 || angle == 180) {
            textAlign = 'center';
        } else if (angle < 180) {
            textAlign = 'left';
        }

        // get label vertical offset info
        // also from drawPointLabels() calcs
        var verticalTextOffset = 0;
        if (angle === 90 || angle === 270) {
            verticalTextOffset = plSize.h / 2;
        } else if (angle > 270 || angle < 90) {
            verticalTextOffset = plSize.h;
        }

        // Calculate bounding box based on textAlign
        var labelTop = pointLabelPosition.y - verticalTextOffset - labelPadding;
        var labelHeight = 2*labelPadding + plSize.h;
        var labelBottom = labelTop + labelHeight;

        var labelWidth = plSize.w + 2*labelPadding;
        var labelLeft;
        switch (textAlign) {
        case 'center':
          var labelLeft = pointLabelPosition.x - labelWidth/2;
          break;
        case 'left':
          var labelLeft = pointLabelPosition.x - labelPadding;

          break;
        case 'right':
          var labelLeft = pointLabelPosition.x - labelWidth + labelPadding;
          break;
        default:
          console.log('ERROR: unknown textAlign '+textAlign);
        }
        var labelRight = labelLeft + labelWidth;

        // Render a rectangle for testing purposes
        ctx.save();
        ctx.strokeStyle = 'red';
        ctx.lineWidth = 1;
        ctx.strokeRect(labelLeft, labelTop, labelWidth, labelHeight);
        ctx.restore();

        // compare to the current click
        if (mouseX >= labelLeft && mouseX <= labelRight && mouseY <= labelBottom && mouseY >= labelTop) {
            alert(scale.pointLabels[i]+' clicked');
            // Break loop to prevent multiple clicks, if they overlap we take the first one.
            break;
        }
    }
};

JSFiddle here:

https://jsfiddle.net/simoncoggins/7r08uLk9/

The downside of this approach is it that it will break if the core labelling implementation changes in the future. It would be better if the library separated the calculation of label position from its rendering and started exposing the position info via the API. Then this solution could be greatly simplified and would be more robust to library changes.

I’ve opened a ticket offering to make that change here:

https://github.com/chartjs/Chart.js/issues/6549

Please comment on that issue if it would be useful to you.