How can I force a zoom to a certain zoomLevel?

[cintiapinto]

I have a bar graph in which the user zooms in and out using buttons (+/-) and I want each time they zoom to take them to the next "breakpoint".

For example, they're currently seeing a graph zoomed in to 100 years:
-> clicking + would ZOOM IN to 50 years
-> clicking - would ZOOM OUT to 250 years

I can programatically zoom in using the zoom function, but I am struggling to find a correlation between the x passed into the zoom function and the final zoomLevel

How exactly should the x value be calculated?

For the example I gave above, from trial and error, I worked out that:
-> chart.zoom({ x: 2 }, true, "zoom") zooms in to 50 years from 100
-> chart.zoom({ x: -2 }, true, "zoom") zooms out to 250 years from 100

Even though I found those numbers by chance, I still can't see how they correlate to the final value I want!

I would love some help on how to make a graph zoom in to a certain number of bars.

Thank you!