Well, detaching the legend does allow for for the items to wrap but then it's next to impossible to determine where the legend should show up by default without landing on top of important trend lines. There's got to be a way to manipulate an attached legend. I did see a way to create a custom legend in QML https://doc.qt.io/qt-5/qtcharts-qmlcustomlegend-example.html but I'm not using QML. Everything I have is built in creator & C++.

Apparently it is virtually impossible to do what I want because the QTextBlock itself hardcodes a QTextLayout in its ::layout() method (see here). Moreover, it seems that every hook we could use is only available in the private classes... Now, the problem is that it prevents everybody wanting to implement custom behavior to create anything based on the QTextDocument and its associates. I wonder how the people who created text editors got away with this (probably implemented everything from scratch). Well, at the end, the only thing I found that could work was to manually insert unicode U+002028 (Line Separator) wherever I want using QTextCursor, which is terrible because everytime the editor area width changes I have to remove the old ones and insert new ones... As I'm not happy with the solution I found, I'll not mark this question as solved.

@chrisadams I think so, I ended up reacting to a signal (from the collection of 'machines') instead of using a timer and I don't have an issue anymore!