You can freely delete yanchor 1.0 and it will do just the same. That's the problem, all animations seem to ignore manual anchors and use only default ones. While I need non default ones 
Does anyone consult the good, old Doc? Like ever...I was about to write the above, but when going for the link, noticed that it's not mentioned there

You need to use floats, like I have in my example. In case of ints, Ren'Py will use position, in case of floats, it will interpolate the coords based on the dimensions of the outer container (size of the displayable in this case). So at anchor .0 (you don't need to write 0.0 in Python) and yzoom: bottom will move. With .5, both bottom and top will "move" (outward movement from center) and with 1.0 the top will be moving.
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Can I close #107? Or are there still issues?