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

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

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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.

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

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