Accepted answer

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:

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