This is a work around not a perfect solution but it should do as you require:
For each column create a small Text Object (I use with a 'Help' style Question mark as the image) and then simply position it in the header of the column and enter the help text you require in it's Object Help box under the Caption tab.
This gives you total flexibility. To ensure they always stay visible set the chart to be the bottom layer and the help boxes to be the top layer.
Hope that helps,
Matt - Visual Analytics Ltd