Remove format
from your dataLabels options. Then you need to properly calculate where put that label. plotY will be undefined, since there is no value, right?
formatter: function () {
var str;
if (this.y !== null) {
return this.y + '%';
} else {
var chart = this.series.chart,
categoryWidth = chart.plotHeight / chart.xAxis[0].categories.length,
offset = (this.point.x + 1) * categoryWidth - this.point.pointWidth + 3; //
chart.renderer.text('n/a', chart.plotLeft, chart.plotTop + offset).add();
}
return false;
},