We have a set of a data that contains counts of instances of events. These can only be integers. When we display data that has a high enough yValue the yAxis labels are integers
Set the allowDecimals option in the y axis to false in order to prevent non integer tick marks from being displayed:
allowDecimals
yAxis: { allowDecimals: false, labels: { style: { fontSize: '9px', width: '175px' } }, title: { text: '' } }
Here is a demonstration: http://jsfiddle.net/sBC9K/