fix the position of the tooltip on the top of each groups of bars

[ibrahim-lahribli]

Hi,
I'm working on this example where we need to have a common tooltip for each group of bars.
the issue I have currently is to calculate the position of the tooltip to have it on the top of each group.
We use a custom tooltip with VictoryVoronoiContainer to group the bars on one tooltip.
you will find below an example of what we have achieved for now please if you can help to finish the job
https://jsfiddle.net/btv60xor/7/
Thanks you