Chart.js : straight lines instead of curves

I'm using Chart.JS to plot a dataset,

However I got a smooth effect !

Here is the curve I've got :

Here is my code :

function plotChart(data, labels) {

    var lineChartData = {
        "datasets": [{
            "data": data,
            "pointStrokeColor": "#fff",
            "fillColor": "rgba(220,220,220,0.5)",
            "pointColor": "rgba(220,220,220,1)",
            "strokeColor": "rgba(220,220,220,1)"
        }],
        "labels": labels
    };

    var myLine = new Chart(document.getElementById("canvas").getContext("2d")).Line(lineChartData);

}

How can I have straight lines instead of curves ?

Thank you

Solution for Version 1 (old charts version)

According to documentation on chartjs.org

you can set the 'bezierCurve' in options and pass it in when you create the chart.

bezierCurve: false

eg:

var options = {
    //Boolean - Whether the line is curved between points
    bezierCurve : false
};
var myLine = new Chart(document.getElementById("canvas").getContext("2d")).Line(lineChartData, options);

Update for version 2

According to updated documentation for Line Configuration in global options

Name 	    Type 	Default Description
tension 	Number 	0.4 	Default bezier curve tension. Set to 0 for no bezier curves.

eg:

var options = {
    elements: {
        line: {
            tension: 0
        }
    }
};

And also directly in the Dataset Structure by setting lineTension to 0 (zero).

Property 	    Type 	Usage
lineTension 	Number 	Bezier curve tension of the line. Set to 0 to draw straightlines. 
                        This option is ignored if monotone cubic interpolation is used. 
                        Note This was renamed from 'tension' but the old name still works.

An example data object using these attributes is shown below.

var lineChartData = {
    labels: labels,
    datasets: [
        {
            label: "My First dataset",
            lineTension: 0,           
            data: data,
        }
    ]
};

You can use lineTension option to set the desired curve. Set 0 for straight lines. You can give a number between 0-1

data: {
    datasets: [{
        lineTension: 0
    }]
}

I have used lineTension to set the smoothness of the curve.

> From the docs: lineTension receives a number, Bezier curve tension of the line. Set to 0 to draw straight lines. This option is ignored if monotone cubic interpolation is used.

Just make sure to test with different values how smooth you want the line.

For example:

var data = {
  labels: ["Jan", "Feb", "Mar"],
  datasets: [{
      label: "Label 1",
      lineTension: 0.2
    }, {
      label: "Stock B",
      lineTension: 0.2
    }

  ]
};

Just to complete version compatibility and to add something to this nice thread here:

Still the same with chart.js v3.x.x
(which is not backwards compatible with v2.x.x -- however, lineTension stays unchanged within
data: { datasets: [{ lineTension: ... )

LineTension for chart.js v3.x.x

Following, you can Run the snippet with 10 buttons to play with different lineTensions (0 to 1) right here:

// for now, let's assume sample data
let sample_data = {
  "Labels" : [
    "2021-08-02",
    "2021-08-03",
    "2021-08-04",
    "2021-08-05",
    "2021-08-06"
  ],
  "Values": [
    6,
    4,
    3,
    8,
    2
  ]
};


// Draw chart
const ctx = document.querySelector('canvas').getContext('2d');

const myLineChart = new Chart(ctx, {
  type: 'line',
  data: {
    labels: sample_data.Labels,
    datasets: [{
      label: 'LineTension Sample',
      data: sample_data.Values,
      lineTension: 0,
      borderColor: 'rgba(0,255,0,1)',
      backgroundColor: 'rgba(0,255,0,0.3)',
      fill: true
    }]
  }
});

function lineTension(event) {
  // Redraw the chart with modified lineTension

  // a bit of 'button-cosmetics' here
  // enabling all buttons
  document.querySelectorAll('button').forEach(element => element.disabled = false);
  // disabling the pressed button
  event.target.disabled = true;

  // setting programmatically the 'lineTension' here
  myLineChart.data.datasets[0].lineTension = parseFloat(event.target.dataset.linetension);

  myLineChart.update();
};

button { color: blue; } button:disabled { color: black; background-color: rgba(0,255,0,0.3); }

<script src="https://cdn.jsdelivr.net/npm/chart.js"></script>
<!-- gets you the latest version of Chart.js, now at v3.5.0 -->

<button onclick="lineTension(event)" data-linetension="0" disabled>0</button>
<button onclick="lineTension(event)" data-linetension="0.1">0.1</button>
<button onclick="lineTension(event)" data-linetension="0.2">0.2</button>
<button onclick="lineTension(event)" data-linetension="0.3">0.3</button>
<button onclick="lineTension(event)" data-linetension="0.4">0.4</button>
<button onclick="lineTension(event)" data-linetension="0.5">0.5</button>
<button onclick="lineTension(event)" data-linetension="0.6">0.6</button>
<button onclick="lineTension(event)" data-linetension="0.7">0.7</button>
<button onclick="lineTension(event)" data-linetension="0.8">0.8</button>
<button onclick="lineTension(event)" data-linetension="0.9">0.9</button>
<button onclick="lineTension(event)" data-linetension="1">1</button>

<canvas width="320" height="240"></canvas>

I think it's been updated to lineTension. Check the docs.