Hello,
you have to select the Interval Shape and then SubSelect the text shape. It should look like in the attached picture.
Then you go to Window->ShapeSheet (don't know the exact english Menu titles, because I use german Visio).
In the ShapeSheet of the text Shape you hav to change the PinY cell from:
=GUARD(IF(User.visIsBracket,Scratch.A1,Sheet.60!Controls.Row_1.Y-Height/2*Scratch.B1)),
deleting the bold parts, to:
=GUARD(IF(User.visIsBracket,Scratch.A1,Sheet.60!Height/2))
The "Sheet.60" will most certainly be another number in your shape.
The textshape is now in the exact middle of the intervalshape, but the text seems still a little bit of center.
To correct that ,too , you have to scroll down in the ShapeSheet to the "Text Block Format" Section an change the formulas in TopMargin and BottomMargin to plain zero: =0
That shall do the trick ;-)
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But one warning: The shape will loose some of its smarts, because in y-direction the text will now longer follow the control point.