score:9

Accepted answer

What you need is tickInterval in xAxis:

xAxis: {
    type: 'datetime',
    tickInterval: 1
}

Updated your jsFiddle here: http://jsfiddle.net/NR8vG/1/

score:10

Datetime axes are based on milliseconds, so an interval of one hour is: 3600 * 1000.

The solution above does not work in charts of type: area.

So if your xAxis is in days you could use:

chart: {
    type: 'area'
},
xAxis: {
    type: 'datetime',
    tickInterval: 24 * 3600 * 1000
}

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