[Solved]-display vertical lines and horizontal lines on plots-Highcharts

The default values for the gridLineWidth is 1 for yAxis and 0 for xAxis, resulting in only the horizontal lines showing. What you need to do is to set the width to 1 for both x and y.

yAxis: {
    gridLineWidth: 1 // The default value, no need to change it
},

xAxis: {
    gridLineWidth: 1 // New value
},

Example on jsfiddle: