[Solved]-How to highlight cells and yaxis ticks on highcharts heatmap hover-Highcharts

You used the boost module which increases performance, but causes some limitations - for example, the inability to change color in states.

To highlight an axis label you need to adjust the findTick function:

function findTick(point, ticks) {
    for (var tickValue in ticks) {
        if (tickValue == point.y) {
            return ticks[tickValue]
        }
    }
}

Live demo: https://jsfiddle.net/BlackLabel/agcper18/

Docs: https://www.highcharts.com/docs/advanced-chart-features/boost-module