Your code is o.k. - at least the way how you use Qwt - so I would expect to find the reason in the syntax of the string you are passing. To be honest I have forgotten about the details of this renderer and - as I said before - it would be best to find the example in the Qt solution package.
But as I'm interested myself I will try to write an example using MathML labels, so that it won't get lost anymore.
Uwe
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