It looks a very nice feature. I'll try and review it soon...

Ok - looks interesting, but there are some problems at a first glance:

• The contour widget doesn't work any more (the self tests don't succeed). I think it is because it drops the key setting, as it uses separate keys for each level.

• Therefore, maybe it would be better not to overwrite the key setting on the document change? Usually Veusz doesn't do self-modifying settings (except for the fit widget). You could hook into getKeyText to generate the automatic text, though I'm not sure how the user would then make the key text empty if they didn't want one.

• I can't get the automatic key to work, if I take the sin example, set a label attribute, then add a key. Nothing seems to show.

• Saving the sin example with the attributes breaks the saved file as SaveDataAttr is put above the ImportString command which makes the dataset. I seem to remember there's some logic to avoid this for dataset tags.

I'll look further at the code later, but thanks!