1. For line chart, you can hide the y axis by setting the value of yAxis.pointOnColumn, yAxis.showLabel option to false as shown below.

var options = {
    ...
    yAxis: {
        pointOnColumn: false,
        showLabel: false
    },
    ...
}

2. Can control the thickness of lines by giving series.pointWidth option as below, but there is no way to change the line weight of specific series group to different.

var options = {
    ...
    series: {
        pointWidth: 1
    },
    ...
}

Git – Book (git-scm.com)

[IntelliJ] Git을 이용한 Commit & Push 하기 및 gitignore 파일 생성 (tistory.com)

Edit fiddle – JSFiddle – Code Playground