You are right - there you should use load event to add extra text for exported image: http://jsfiddle.net/3bQne/88/
chart.exportChart(null, {
chart: {
backgroundColor: '#FFFFFF',
width: 972,
height: 480,
events: {
load: function () {
this.renderer.text('Filtered', 5, 10)
.attr({
rotation: 0
})
.css({
color: '#4572A7',
fontSize: '8px',
fontStyle: 'italic'
})
.add();
}
}
}
});