Use the text shape instead of the simple text shape.
This has two reasons.

The first one is that it turned out to be impossible (without risking crashes) to use an interface to a plugin if the interface definition is not in the libs.

If I had knew this before, that would have saved me and Jan (thanks again for your help) a lot of time.

Second reason is that the text shape is more flexible than it seemed to me, which means that the functionality that chart labels don't support can be turned off.