The behavior is designed such that zooming is limited upto a certain region, so the user doesn’t end up zooming into a blank-region (region with no dataPoints). To zoom into a certain region, there should be a minimum of 3-4 dataPoints.
In your case, zoom doesn’t work as its getting invalidated over axisY. However zoomType: ‘x’ works fine in your case.
We will reconsider this behavior for future releases.