You can use second yAxis, so you will be able to set extremes, which will work as your offset, see: http://jsfiddle.net/3bQne/921/
var chart = new Highcharts.Chart({
chart: {
renderTo: 'container'
},
xAxis: {
categories: ['01/02/2012', '01/03/2012', '01/04/2012', '01/05/2012', '01/06/2012', '01/07/2012']
},
yAxis: [{
// default options
}, {
opposite: true,
min: 0,
max: 5
}],
series: [{
type: 'column',
data: [29.9, 71.5, 106.4, 129.2, 144.0, 176.0]
}, {
type: 'line',
data: [4, 4, 4, 4, 4, 4],
yAxis: 1
}]
});