If your xAxis is set to datetime, then your JSON needs JS timestamps (Unixtime*1000). I'd suggest you use categories as xAxis:
xAxis: {
categories: ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec']
},
Then you also have to change your JSON to deliver 12 values sorted from January to December.