How to put rounded corners on a Chart.js Bar chart

Question

I have created a bar chart in chart.js using the below code. However I want to give the bars rounded corners instead of edged ones at the top of the bars. I can't find any way to do this using the global settings of chart.js. Is there any way to achieve the effect I want?

var barContext = document.getElementById("canvas").getContext("2d");

var barGradientFirst = barContext.createLinearGradient(0, 0, 0, 450);
barGradientFirst.addColorStop(0, 'rgba(112,122,157, 0.1)');   
barGradientFirst.addColorStop(1, 'rgba(112,122,157, 1)');

var barGradientSecond = barContext.createLinearGradient(0, 0, 0, 450);
barGradientSecond.addColorStop(0, 'rgba(151,122,208, 0.1 )');  
barGradientSecond.addColorStop(1, 'rgba(151,122,208, 1)');


var barChartData = {
labels: ["High", "Med", "Low", "None"],
datasets : [
    {
        fillColor : barGradientFirst,
        strokeColor: "rgb(112,122,200)",
        data: [30, 40, 70, 90]
    }, {
        fillColor : barGradientSecond,
        strokeColor: "rgba(220,100,80,0.8)",
        data: [50, 60, 65, 20]
    }]  
};

new Chart(barContext).Bar(barChartData, {
    responsive : true,
    scaleOverride : true,
    scaleBeginAtZero : true,
    scaleSteps : 2,
    scaleLineWidth: 3,
    scaleStepWidth : 50,
    scaleShowLabels : true,
    scaleShowVerticalLines: false,
    scaleShowHorizontalLines: false,
    scaleFontSize: 30,
    barValueSpacing : 40,
    barDatasetSpacing : 3,
    scaleLabel: "<%= value + '%' %>"
});

Answer 1

As pointed out in https://stackoverflow.com/a/68778432/360067, this is natively implemented in v3.

---

Original Answer

Here is how you extend Chart.js to draw a bar chart with rounded corners.

Chart.types.Bar.extend({
    name: "BarAlt",
    initialize: function (data) {
        Chart.types.Bar.prototype.initialize.apply(this, arguments);
            
        if (this.options.curvature !== undefined && this.options.curvature <= 1) {
            var rectangleDraw = this.datasets[0].bars[0].draw;
            var self = this;
            var radius = this.datasets[0].bars[0].width * this.options.curvature * 0.5;

            // override the rectangle draw with ours
            this.datasets.forEach(function (dataset) {
                dataset.bars.forEach(function (bar) {
                    bar.draw = function () {
                        // draw the original bar a little down (so that our curve brings it to its original position)
                        var y = bar.y;
                        // the min is required so animation does not start from below the axes
                        bar.y = Math.min(bar.y + radius, self.scale.endPoint - 1);
                        // adjust the bar radius depending on how much of a curve we can draw
                        var barRadius = (bar.y - y);
                        rectangleDraw.apply(bar, arguments);

                        // draw a rounded rectangle on top
                        Chart.helpers.drawRoundedRectangle(self.chart.ctx, bar.x - bar.width / 2, bar.y - barRadius + 1, bar.width, bar.height, barRadius);
                        ctx.fill();

                        // restore the y value
                        bar.y = y;
                    }
                })
            })
        }
    }
});


var lineChartData = {
    labels: ["January", "February", "March", "April", "May", "June"],
    datasets: [
        {
            fillColor: "#79D1CF",
            strokeColor: "#79D1CF",
            data: [60, 80, 81, 56, 55, 40]
        },
        {
            fillColor: "#D1CF79",
            strokeColor: "#D1CF79",
            data: [34, 43, 43, 12, 65, 65]
        }
    ]
};

var ctx = document.getElementById("myChart").getContext("2d");
var myLine = new Chart(ctx).BarAlt(lineChartData, {
    // 0 (flat) to 1 (more curvy)
    curvature: 1
});

You could simplify the code a bit if you don't need the animation.

---

Fiddle - http://jsfiddle.net/0dzp3jxw/

---

Answer 2

Starting on Chart.js v3 you can just specify borderRadius: NUMBER on the dataset.

new Chart(document.querySelector('canvas').getContext('2d'), {
        type: 'bar',
        data: {
            labels: ['1','2','3','4','5','6'],
            datasets: [{
                label: 'Sales',
                data: [20,30,10,20,30,10],
                fill: true,
                borderRadius: 15,
                borderColor: '#5eb8ff',
                backgroundColor: '#5eb8ff',
                datalabels: {
                    display: false
                }
            }]
        }
    });

<!DOCTYPE html>
<html>
<head>
  <title></title>
</head>
<body>
  <canvas></canvas>
  <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/chart.js@3.0.0/dist/chart.min.js"></script>
</body>
</html>