KB Heatmap v2

HTML

<div><textarea id=source cols=60 rows=15>Enter text to see result.</textarea></div>
<div><canvas id=heatmap width=500 height=200></canvas></div>

JavaScript

function updateHeatmap(text) {
    let keys = [
        { label: "q", row: 0, col: 0, home: false },
        { label: "w", row: 0, col: 1, home: false },
        { label: "e", row: 0, col: 2, home: false },
        { label: "r", row: 0, col: 3, home: false },
        { label: "t", row: 0, col: 4, home: false },
        { label: "y", row: 0, col: 5, home: false },
        { label: "u", row: 0, col: 6, home: false },
        { label: "i", row: 0, col: 7, home: false },
        { label: "o", row: 0, col: 8, home: false },
        { label: "p", row: 0, col: 9, home: false },
        { label: "a", row: 1, col: 0, home: true },
        { label: "s", row: 1, col: 1, home: true },
        { label: "d", row: 1, col: 2, home: true },
        { label: "f", row: 1, col: 3, home: true },
        { label: "g", row: 1, col: 4, home: false },
        { label: "h", row: 1, col: 5, home: false },
        { label: "j", row: 1, col: 6, home: true },
        { label: "k", row: 1, col: 7, home: true },
        { label: "l", row: 1, col: 8, home: true },
        { label: ";", row: 1, col: 9, home: true },
        { label: "z", row: 2, col: 0, home: false },
        { label: "x", row: 2, col: 1, home: false },
        { label: "c", row: 2, col: 2, home: false },
        { label: "v", row: 2, col: 3, home: false },
        { label: "b", row: 2, col: 4, home: false },
        { label: "n", row: 2, col: 5, home: false },
        { label: "m", row: 2, col: 6, home: false },
        { label: ",", row: 2, col: 7, home: false },
        { label: ".", row: 2, col: 8, home: false },
        { label: "/", row: 2, col: 9, home: false },
    ]
	var charCounts = countChars(text)
    var keyWeights = weighKeys(charCounts, keys)

    let heatmapCanvas = document.getElementById("heatmap")
    let ctx = heatmapCanvas.getContext("2d")
    let keySize = heatmapCanvas.width / 12

    ctx.clearRect(0, 0, heatmapCanvas.width, heatmapCanvas.height)
    for (let key of keys) {
        let x = (1 + key.col +...