JSFiddle - React, Tailwind, and code Playground

HTML

<body></body>

CSS

svg {
    font: 10px sans-serif;
}
.line {
    fill: none;
    stroke: #000;
    stroke-width: 1.5px;
}
.axis path, .axis line {
    fill: none;
    stroke: #000;
    shape-rendering: crispEdges;
}

JavaScript

var n = 40,
    random = d3.random.normal(0, .2),
    data = []

var margin = {
    top: 20,
    right: 20,
    bottom: 20,
    left: 40
},
width = 960 - margin.left - margin.right,
    height = 500 - margin.top - margin.bottom;

var x = d3.scale.linear()
    .domain([0, n - 1])
    .range([0, width]);

var y = d3.scale.linear()
    .domain([0, 60])
    .range([height, 0]);

var line = d3.svg.line()
    .x(function (d, i) {
    return x(i);
})
    .y(function (d, i) {
    return y(d);
});

var svg = d3.select("body").append("svg")
    .attr("width", width + margin.left + margin.right)
    .attr("height", height + margin.top + margin.bottom)
    .append("g")
    .attr("transform", "translate(" + margin.left + "," + margin.top + ")");

svg.append("defs").append("clipPath")
    .attr("id", "clip")
    .append("rect")
    .attr("width", width)
    .attr("height", height);

svg.append("g")
    .attr("class", "x axis")
    .attr("transform", "translate(0," + y(0) + ")")
    .call(d3.svg.axis().scale(x).orient("bottom"));

var path = svg.append("g")
    .attr("clip-path", "url(#clip)")
    .append("path")
    .datum(data)
    .attr("class", "line")
    .attr("d", line);

socket = new WebSocket("wss://wss.redditmedia.com/thebutton?h=61e3651cd8fac82c0c03d8d9cc5f583e2200d8d7&e=1427996771")

socket.onmessage = function (data2) {
    data.push(JSON.parse(data2.data).payload.seconds_left)
    // redraw the line, and slide it to the left
    path.attr("d", line)
        .attr("transform", null)

    if (data.length > 40) {
        data.shift()
        path.transition()
            .duration(500)
            .ease("linear")
            .attr("transform", "translate(" + x(-1) + ",0)");
    }

}