Highchart Heatmap shows index number in YAxis

Question

Heatmap shows index in YAxis instead of Category Name. Is this bug in Highchart?

function getPointCategoryName(point, dimension) {
    var series = point.series,
        isY = dimension === 'y',
        axis = series[isY ? 'yAxis' : 'xAxis'];
    return axis.categories[point[isY ? 'y' : 'x']];
}

Highcharts.chart('container', {

    chart: {
        type: 'heatmap',
        marginTop: 40,
        marginBottom: 80,
        plotBorderWidth: 1
    },


    title: {
        text: 'Sales per employee per weekday'
    },

    xAxis: {
        categories: ['Alexander', 'Marie', 'Maximilian', 'Sophia', 'Lukas', 'Maria', 'Leon', 'Anna', 'Tim', 'Laura']
    },

    yAxis: {
        categories: [
            'Hello-Yaxis-1',
          'Hello-Yaxis-2',
          'Hello-Yaxis-3',
          'Hello-Yaxis-4',
          'Hello-Yaxis-5',
          'Hello-Yaxis-6',
          'Hello-Yaxis-7',
          'Hello-Yaxis-8',
          'Hello-Yaxis-9',
          'Hello-Yaxis-10',
          'Hello-Yaxis-11',
          'Hello-Yaxis-12',
          'Hello-Yaxis-13',
          'Hello-Yaxis-14',
          'Hello-Yaxis-15',
          'Hello-Yaxis-16',
          'Hello-Yaxis-17',
          'Hello-Yaxis-18',
          'Hello-Yaxis-19',
          'Hello-Yaxis-20'
        ],
        title: null,
        reversed: true
    },

    accessibility: {
        point: {
            descriptionFormatter: function (point) {
                var ix = point.index + 1,
                    xName = getPointCategoryName(point, 'x'),
                    yName = getPointCategoryName(point, 'y'),
                    val = point.value;
                return ix + '. ' + xName + ' sales ' + yName + ', ' + val + '.';
            }
        }
    },

    colorAxis: {
        min: 0,
        minColor: '#FFFFFF',
        maxColor: Highcharts.getOptions().colors[0]
    },

    legend: {
        align: 'right',
        layout: 'vertical',
        margin: 0,
        verticalAlign: 'top',
        y: 25,
        symbolHeight: 280
    },

    tooltip: {
        formatter: function () {
            return '<b>' + getPointCategoryName(this.point, 'x') + '</b> sold <br><b>' +
                this.point.value + '</b> items on <br><b>' + getPointCategoryName(this.point, 'y') + '</b>';
        }
    },

    series: [{
        name: 'Sales per employee',
        borderWidth: 1,
        data: [[0, 0, 10], [0, 1, 19], [0, 2, 8], [0, 3, 24], [0, 4, 67], [1, 0, 92], [1, 1, 58], [1, 2, 78], [1, 3, 117], [1, 4, 48], [2, 0, 35], [2, 1, 15], [2, 2, 123], [2, 3, 64], [2, 4, 52], [3, 0, 72], [3, 1, 132], [3, 2, 114], [3, 3, 19], [3, 4, 16], [4, 0, 38], [4, 1, 5], [4, 2, 8], [4, 3, 117], [4, 4, 115], [5, 0, 88], [5, 1, 32], [5, 2, 12], [5, 3, 6], [5, 4, 120], [6, 0, 13], [6, 1, 44], [6, 2, 88], [6, 3, 98], [6, 4, 96], [7, 0, 31], [7, 1, 1], [7, 2, 82], [7, 3, 32], [7, 4, 30], [8, 0, 85], [8, 1, 97], [8, 2, 123], [8, 3, 64], [8, 4, 84], [9, 0, 47], [9, 1, 114], [9, 2, 31], [9, 3, 48], [9, 4, 91],
        [9, 19, 999999]
        ],
        dataLabels: {
            enabled: true,
            color: '#000000'
        }
    }],

    responsive: {
        rules: [{
            condition: {
                maxWidth: 500
            },
            chartOptions: {
                yAxis: {
                    labels: {
                        formatter: function () {
                            return this.value.charAt(0);
                        }
                    }
                }
            }
        }]
    }

});

.highcharts-figure,
.highcharts-data-table table {
    min-width: 360px;
    max-width: 800px;
    margin: 1em auto;
}

.highcharts-data-table table {
    font-family: Verdana, sans-serif;
    border-collapse: collapse;
    border: 1px solid #ebebeb;
    margin: 10px auto;
    text-align: center;
    width: 100%;
    max-width: 500px;
}

.highcharts-data-table caption {
    padding: 1em 0;
    font-size: 1.2em;
    color: #555;
}

.highcharts-data-table th {
    font-weight: 600;
    padding: 0.5em;
}

.highcharts-data-table td,
.highcharts-data-table th,
.highcharts-data-table caption {
    padding: 0.5em;
}

.highcharts-data-table thead tr,
.highcharts-data-table tr:nth-child(even) {
    background: #f8f8f8;
}

.highcharts-data-table tr:hover {
    background: #f1f7ff;
}

<script src="https://code.highcharts.com/highcharts.js"></script>
<script src="https://code.highcharts.com/modules/heatmap.js"></script>
<script src="https://code.highcharts.com/modules/exporting.js"></script>
<script src="https://code.highcharts.com/modules/export-data.js"></script>
<script src="https://code.highcharts.com/modules/accessibility.js"></script>

<figure class="highcharts-figure">
    <div id="container"></div>
    <p class="highcharts-description">
        Heatmap showing employee data per weekday. Heatmaps are commonly used to
        visualize hot spots within data sets, and to show patterns or correlations.
        Due to their compact nature, they are often used with large sets of data.
    </p>
</figure>

Answer 1

It's not a bug. Notice that the tickInterval in your case is 2, so the next label should be 'Hello-Yaxis-21' which is not defined - that's why Highcharts render the next value as a count number - it is the default behavior.

See: https://jsfiddle.net/BlackLabel/sqw78ugj/