After inspecting the used example and the real code, I can assume that the issue may occur due to the text length and the set width and height of the diagram box.
I can suggest setting only one of the TextFields instead of all of them to observe if there is only one item it will be centred:
layout.append(new dataviz.diagram.TextBlock({
text: dataItem.TextFields[0],
fontFamily: "DejaVu Sans",
color: "black"
If the issue still occurs, please provide a runnable example, as this will help us inspect it locally and provide a suggestion best suited for it.