Line chart labels using textPath

I liked the labelling on this chart from The Financial Times, so recreated it in D3 using SVG's textPath, and using a Makeover Monday dataset.

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<!DOCTYPE html>

<head>
    <meta charset="utf-8">
    <link href="https://fonts.googleapis.com/css?family=Oxygen" rel="stylesheet">
    <script src="https://d3js.org/d3.v4.min.js"></script>
    <style>
        body {
            font-family: 'Oxygen', sans-serif;
            font-size: 12px
            margin: 0;
            top: 0;
            right: 0;
            bottom: 0;
            left: 0;
            background: #222222
        }

.line-app {
            fill: none;
            stroke-width: 4px;
        }
    </style>
</head>

let appData = [
    {
      "date": "1/03/2015",
      "app": "Snapchat",
      "percentage": 0.11
    },
    {
      "date": "1/09/2015",
      "app": "Snapchat",
      "percentage": 0.17
    },
    {
      "date": "1/03/2016",
      "app": "Snapchat",
      "percentage": 0.24
    },
    {
      "date": "1/09/2016",
      "app": "Snapchat",
      "percentage": 0.35
    },
    {
      "date": "1/03/2017",
      "app": "Snapchat",
      "percentage": 0.39
    },
    {
      "date": "1/09/2017",
      "app": "Snapchat",
      "percentage": 0.47
    },
    {
      "date": "1/03/2015",
      "app": "Instagram",
      "percentage": 0.29
    },
    {
      "date": "1/09/2015",
      "app": "Instagram",
      "percentage": 0.29
    },
    {
      "date": "1/03/2016",
      "app": "Instagram",
      "percentage": 0.23
    },
    {
      "date": "1/09/2016",
      "app": "Instagram",
      "percentage": 0.24
    },
    {
      "date": "1/03/2017",
      "app": "Instagram",
      "percentage": 0.23
    },
    {
      "date": "1/09/2017",
      "app": "Instagram",
      "percentage": 0.24
    },
    {
      "date": "1/03/2015",
      "app": "Facebook",
      "percentage": 0.12
    },
    {
      "date": "1/09/2015",
      "app": "Facebook",
      "percentage": 0.13
    },
    {
      "date": "1/03/2016",
      "app": "Facebook",
      "percentage": 0.15
    },
    {
      "date": "1/09/2016",
      "app": "Facebook",
      "percentage": 0.13
    },
    {
      "date": "1/03/2017",
      "app": "Facebook",
      "percentage": 0.11
    },
    {
      "date": "1/09/2017",
      "app": "Facebook",
      "percentage": 0.09
    },
    {
      "date": "1/03/2015",
      "app": "Twitter",
      "percentage": 0.21
    },
    {
      "date": "1/09/2015",
      "app": "Twitter",
      "percentage": 0.18
    },
    {
      "date": "1/03/2016",
      "app": "Twitter",
      "percentage": 0.16
    },
    {
      "date": "1/09/2016",
      "app": "Twitter",
      "percentage": 0.13
    },
    {
      "date": "1/03/2017",
      "app": "Twitter",
      "percentage": 0.11
    },
    {
      "date": "1/09/2017",
      "app": "Twitter",
      "percentage": 0.07
    }
   ]

const formatDate = d3.timeFormat("%b %y") // Mar 17
        const parseDate = d3.timeParse("%d/%m/%Y") // 1/03/2017

function formatPercentage(n) { return Math.round(n * 100) + "%"; }

appData.forEach(function (d) {
            d.parsedDate = parseDate(d.date)
        })

let nestedData = d3.nest()
            .key(function (d) { return d.app; })
            .entries(appData)

//////////////////////////////////////////////////////////////////

const width = 700
        const height = 500

const margin = { "top": 50, "bottom": 50, "left": 50, "right": 150, }

let xScale = d3.scaleTime()
            .domain(d3.extent(appData, function (d) { return d.parsedDate; }))
            .range([0, width])

let yScale = d3.scaleLinear()
            .domain([0, 0.5])
            .range([height, 0])

let xAxis = d3.axisBottom(xScale)
            
        let yAxis = d3.axisLeft(yScale)
            .ticks(5)

let colour = d3.scaleOrdinal()
            .domain(["Twitter", "Snapchat", "Facebook", "Instagram"])
            .range(["#00aced", "#fffc00", "#3b5998", "#cd486b"])

let backgroundColour = "#222222"

let line = d3.line()
            .x(function (d) { return xScale(d.parsedDate); })
            .y(function (d) { return yScale(d.percentage); })
            .curve(d3.curveCardinal.tension(0.5));

let svg = d3.select("body").append("svg")
            .attr("width", width + margin.left + margin.right)
            .attr("height", height + margin.top + margin.bottom)

let g = svg.append("g")
            .attr("transform", "translate(" + margin.left + "," + margin.top + ")")

//////////////////////////////////////////////////////////////////

let yAxixBoxDate = new Date("1/1/2017")
        let yAxixBoxWidth = 30
        let yAxixBoxHeight = 20

let axes = g.append("g").attr("class", "axes")

let yAxisG = axes.append("g")
            .attr("transform", "translate(0,0)")
            .call(yAxis)

let xAxisG = axes.append("g")
            .attr("transform", "translate(0," + height + ")")

let xTicks = xAxisG.selectAll(".tick")
            .data(nestedData[0].values )
            .enter()
            .append("g")
            .attr("class", "tick")
            .attr("transform", function(d){ return "translate(" + xScale(d.parsedDate) + ",0)"; })

xTicks.append("text")
            .text(function (d) { return formatDate(d.parsedDate); })
            .attr("y", 18)
            .style("text-anchor", "middle")

xTicks.append("line")
            .attr("y1", -height)
            .attr("y2", 1)

yAxisG.selectAll(".tick").selectAll("text")
            .text(function (d) { return formatPercentage(d); })
            .attr("x", xScale(new Date("1/1/2017")))
            .style("text-anchor", "middle")

yAxisG.selectAll(".tick").selectAll("line")
            .attr("x1", 0)
            .attr("x2", width)

yAxisG.selectAll(".tick").append("rect")
            .attr("x", xScale(yAxixBoxDate) - (yAxixBoxWidth/2))
            .attr("y", -(yAxixBoxHeight/2))
            .attr("width", yAxixBoxWidth)
            .attr("height", yAxixBoxHeight)
            .style("fill", backgroundColour)
        
        yAxisG.selectAll(".tick").selectAll("text")
            .text(function (d) { return formatPercentage(d); })
            .attr("x", xScale(new Date("1/1/2017")))
            .style("text-anchor", "middle")
            .raise()

axes.selectAll(".domain").remove()

axes.selectAll(".tick").selectAll("line")
            .style("stroke", "grey")
            .style("stroke-dasharray", "2,2")
            .style("stroke-linecap", "round")

axes.selectAll(".tick").selectAll("text")
            .style("fill", "grey")

//////////////////////////////////////////////////////////////////

let lines = g.append("g").attr("class", "lines")

let app = lines.selectAll("g")
            .data(nestedData)
            .enter()
            .append("g")
            .attr("id", function (d) { return d.key })

lines.select("#Snapchat").raise()

app.append("path")
            .datum(function (d) { return d.values; })
            .attr("d", line)
            .attr("class", "line-app")
            .attr("id", function(d){ return "line-app-" + d[0].app })
            .style("stroke", function (d) { return colour(d[0].app); })
            .style("stroke-width", "4px")
            .style("fill", "none")

app.selectAll("circle")
            .data(function (d) { return d.values; })
            .enter()
            .append("circle")
            .attr("cx", function(d){ return xScale(d.parsedDate) })
            .attr("cy", function(d){ return yScale(d.percentage) })
            .attr("r", 4)
            .style("fill", function (d) { return colour(d.app); })
            .style("stroke", backgroundColour)
            .style("stroke-width", 2)

app.append("text")
            .attr("dy", function(d){ return d.key == "Twitter" ? 17 : -7; })
            .append("textPath")
            .text(function (d) { return d.key })
            .attr("xlink:href", function(d){ return "#" + "line-app-" + d.key; }) 
            .attr("startOffset", "99%")
            .attr("text-anchor", "end")
            .style("fill", function (d) { return colour(d.key); })

//////////////////////////////////////////////////////////////////

</script>
</body>

https://d3js.org/d3.v4.min.js