in the last versions of ChartJS minIndex and maxIndex are not link to de visible points, they are linked to X axis position visible, I found the same problem. But I found 2 properties that I used to solve it: chart._metasets[0].controller._drawStart ; chart._metasets[0].controller._drawCount ; thats shows the first point visible Index and the number of visible points, both needs some corrections, but , if you are here, You will fix it.

PD. I use Zoom in my code and still work