[Solved]-Show only the first and last y-axis label on a graph highcharts-Highcharts

you can find the 4 edge points of one serie in this way:

const pointssortedbyy = serie.points.sort((p1, p2) => p2.y - p1.y);
const top = pointssortedbyy[0];
const bottom = pointssortedbyy[pointssortedbyy.length - 1];

const pointssortedbyx = serie.points.sort((p1, p2) => p2.x - p1.x);
const first = pointssortedbyx[0];
const last = pointssortedbyx[pointssortedbyx.length - 1];

then you can print it:

[top,bottom, first, last].foreach(function (point) {
    point.update({
        marker: {
            enabled: true,
            fillcolor: 'green'
        },
        datalabels: {
            enabled: true,
            color: 'green'
        }
    });
},this);

and finally repeat the same operation for each series:

chart1.series.foreach(function (serie) {
    printtopandbottom(serie);
}, this);

this is the result:

https://jsfiddle.net/7ue8ow0b/