Thanks for the reply.
I am afraid that the data type change doesn't help. When I tried to diff the raw Chart1.xml, I found that problem was due to an extra node added under <c:catAx>:
<c:txPr> <a:bodyPr/> <a:lstStyle/>
This catAx has nothing to do with the scaling I was working on. That is located under valAx.
After I removed the green part and packed it back to the xlsx file, it worked and the scaling was changed.
The whole node: