You can do this by telling the xAxis how many points to show regardless of how much data you have. This is a bit of a hack as the xAxis has no values exactly - but, it does have index values for each category starting with 0. So, you have 21 categories which means max index is 20. Then you xAxis property would look like:

xAxis: {
    categories: ['08:00', '08:30', '09:00', '09:30', '10:00',
                '10:30', '11:00', '11:30', '12:00', '12:30',
                '13:00', '13:30', '14:00', '14:30', '15:00',
                '15:30', '16:00', '16:30', '17:00', '17:30',
                '18:00'],
    min: 0,
    max: 20
},