How to show only integer values on yAxis of HighChart?

Set the allowDecimals option in the y axis to false in order to prevent non integer tick marks from being displayed:

yAxis: {
    allowDecimals: false,
    labels: {
        style: {
            fontSize: '9px',
            width: '175px'
        }
    },
    title: {
        text: ''
    }
}

Here is a demonstration: http://jsfiddle.net/sBC9K/