The diagram itself doesn't have any problem

Actually it does, as you have set the value x=10 two times. Therefore, it's not mathematically a function and this maybe is preventing it to render the tooltips correctly. I changed it and the tooltips seem to work fine (displaying the following information: 'Tokio', y-value in bold and x-value at the top). Example


Caution, extreme hackery ahead: You can create more than one series on a chart so you could do something like this. Using 2 series. Of course the curves are not properly matched but it gives you a starting point.

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