Hi StevenAtHL,
In the case that you describe sounds like point should be positioned relatively to chart area. If you want it to scale properly then you can try to get percentage position by multiplying absolute chart dimensions (in pixel)s by a fraction, for example chart width * 0.5 should always keep the element in the middle of the chart.
Demo: https://jsfiddle.net/BlackLabel/d0x15s4k/
Let me know what do you think about this solution.
Regards!