chart.js

google.charts.load('current', {'packages':['gauge']});
google.charts.setOnLoadCallback(drawChart);
let voc_div = document.getElementById('voc_chart');
let voc = parseInt(voc_div.dataset.level);
let voc_max = parseInt(voc_div.dataset.max);

let nox_div = document.getElementById('nox_chart');
let nox = parseInt(nox_div.dataset.level);
let nox_max = parseInt(nox_div.dataset.max);

function drawChart() {

    let voc_data = google.visualization.arrayToDataTable([
        ['Label', 'Value'],
        ['VOC', voc]
    ]);

    let nox_data = google.visualization.arrayToDataTable([
        ['Label', 'Value'],
        ['NOX', nox]
    ]);

    let voc_options = {
        width: 800, height: 240,
        redFrom: voc_max * .9, redTo: voc_max,
        yellowFrom: voc_max * .75, yellowTo: voc_max * .9,
        max: voc_max,
        minorTicks: 5
    };

    let nox_options = {
        width: 800, height: 240,
        redFrom: nox_max * .9, redTo: nox_max,
        yellowFrom: nox_max * .75, yellowTo: nox_max * .9,
        max: nox_max,
        minorTicks: 5
    };

    let voc_chart = new google.visualization.Gauge(voc_div);
    let nox_chart = new google.visualization.Gauge(nox_div);

    voc_chart.draw(voc_data, voc_options);
    nox_chart.draw(nox_data, nox_options);

    setInterval(function () {
        voc_data.setValue(0, 1, voc + Math.round(voc * .05 * Math.random()));
        voc_chart.draw(voc_data, voc_options);
    }, 150);
    setInterval(function () {
        nox_data.setValue(0, 1, nox + Math.round(nox * .05 * Math.random()));
        nox_chart.draw(nox_data, nox_options);
    }, 150);
}