The Users' guide as always explains everything:
https://www.amibroker.com/guide/drawtools.html
Fibonacci arc
This new drawing tool generates standard Fibonacci-arcs that are controlled by the trend line drawn with a dotted style. To see the properites of the arcs click on the controlling trend line.
Note that arc radius and central point are relative to the controlling trendline and because Fibonacci arcs must be circular regardless of screen size/resolution and zoom factor the position of the arcs may move in date/price domain.
You can't have the cake and it eat too.... you either keep the shape in pixel domain OR in price/bar domain. Not both.
Controlling trendline stays in place (in bar/price domain) when you zoom. But circular arc connecting the ends can't.
Those analysis "techniques" (Fib arcs/Gann/etc) were born when people were drawing charts on paper. They did not have "zoom" feature. The scaling was set once by defining certain bar:price ratio and kept fixed. They talked about "angles" and similar nonsense.
I wrote many times that speaking about angles and fixed shapes has very little meaning when X and Y axes do not use same units. Bar (time) and dollars (price) are NOT same units.