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qwt is an extension of Qt as third party library. It maintains its own mailing list. Please check out there. This might be a faster way to get feedback on the issue.
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Just a guess here: did you try using a model (QStandardItemModel) to store the values and use Qt::ToolTipRole to store the tooltip you want to show and them use QVBarModelMapper to plot it?
Good point, I've read your post with glBegin and glEnd in mind.