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It was my mistake, I was passing the data with a wrong format.
In codepen code above, X and Y data were passed using two different array (the first as labels). I should have passed the XY data as a single, two components array.
There is another codepen in my dashboard (Chart.js Linear Zoom example) that shows the way to beginners like me
Callback for ticks was definitely not the problem.
Source:stackexchange.com