How do I get a chart.js chart to display data in a Node/React web application?

Question

I'm trying to display financial data on a web app using the Chart.js library.

In my Node.js app, a user uploads a csv file. This file gets processed into a pandas dataframe by a flask rest api. The dataframe is saved on the flask server and is accessible via a get request. The get request returns a JSON object with fields Open, High, Low, Close, and Volume. These fields hold arrays.

I have a web socket (socket.io) established between my express server and a react component. My express server pings my flask api looking for the chart data. When the chart data is available (after uploading a file) the socket sends this data to the component. The client socket triggers a component method that creates the chart. A chart appears but has no data points displayed.

The component:

class Canvas extends Component {
    constructor(props){
        super(props);
        this.state = {
            data: false,
            endpoint: 'http://localhost:4000'
        }

        this.establishSocket = this.establishSocket.bind(this);
        this.makeChart = this.makeChart.bind(this);
    }

    componentDidMount() {
        this.establishSocket();
    }

    establishSocket() {
        const { endpoint } = this.state;
        const socket = socketIOClient(endpoint);
        socket.on("ohlcv_data", data => this.makeChart(data.Close));
    }

    makeChart(datapoints) {
        this.setState({ data: datapoints })
        var ctx = document.getElementById("myChart");
        var myChart = new Chart(ctx, {
            type: 'line',
            data: datapoints,
            options: { 
                responsive: true, 
                maintainAspectRatio: false
            }
        });
        console.log(myChart.data);
    }

    render() {
        return (
            <div id="chartContainer">
                <canvas id="myChart"></canvas>
            </div>
        )
    }
}

The charting code has been adapted from the Chart.js docs here:

https://www.chartjs.org/docs/latest/getting-started/usage.html

https://www.chartjs.org/docs/latest/charts/line.html

This is my app with the empty chart:

The console log at the bottom of the makeChart function displays the expected array. This is the output of that log:

My question is, what am I missing in this implementation in order to get the data to appear?

Just a random comment: you might want to consider using a react implementation of chart.js such as npmjs.com/package/react-chartjs-2 — Andrew L
– Andrew L, Commented Sep 12, 2018 at 0:09

cdrn · Accepted Answer · 2018-09-12 00:36:57Z

3

I could be wrong, but it looks like the input data for your linechart is in an incorrect format. Looking here at chartJs' documentation, you either have to specify the x and y of each point individually using an array of objects, or in your case, when passing an array of numbers, specify a labels array.

When the data array is an array of numbers, the x axis is generally a category. The points are placed onto the axis using their position in the array. When a line chart is created with a category axis, the labels property of the data object must be specified.

You can see how to do that here, so your chart data param should look like this:

let chart = new Chart(ctx, {
    type: ...
    data: {
        labels: ['January', 'February', 'March', 'April', 'May', 'June'],
        datasets: ...
    },
});

answered Sep 12, 2018 at 0:36

cdrn

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1 Comment

Austin Mackillop Over a year ago

I think you may be right. I read more carefully this time at the example here and it says: The data property of a dataset for a line chart can be passed in two formats. I am setting the data property of the object itself based on the example of using an array for the data property of a dataset.

Austin Mackillop · Accepted Answer · 2018-09-12 01:49:40Z

Thanks to cdm for putting me in the right direction. I remembered that one of my github repositories has chart.js implementations that my teammates made which replaced the google charts that I was using originally.

They are implemented like so:

makeChart(datapoints) {
    this.setState({ data: datapoints })
    var ctx = document.getElementById("myChart");
    var datapoints = datapoints.slice(0,100)
    var config = {
        type: 'line',
        data: {
            labels: this.linspace(0,datapoints[0],datapoints.length),
            datasets: [{
                data: datapoints,
                label: "Price:",
                borderColor: "#3e95cd",
                fill: false
            }]
        },
        options: {
            title: {
                display: true,
                text: 'WTI Closing Price'
            },
            legend: {
                display: false
            }
        }
    }
    var myChart = new Chart(ctx, config );
}

This configuration format works as expected producing the chart below:

For completeness, for those who may use this exact code, the linspace function is implemented as follows:

precisionRound(number, precision) {
    var factor = Math.pow(10, precision);
    return Math.round(number * factor) / factor;
}

linspace (a, b, n) {
    if (typeof n === 'undefined') n = Math.max(Math.round(b - a) + 1, 1)
    if (n < 2) {
        return n === 1 ? [a] : []
    }
    var i,ret = Array(n)
    n--
    for (i = n;i >= 0;i--) {
        ret[i] = this.precisionRound((i * b + (n - i) * a) / n, 2)
    }
    return ret
}

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