Model-driven app - add chart of an entity - missing fields options in Horizontal ( Category ) Axis Labels

[BellaShiu]

I am adding charts of an entity in PowerApps.

However I met a problem : While selecting fields in Legend Entries ( Series ), all of the custom fields of the entity appears normally, but custom fields in Horizontal ( Category ) Axis Labels only appears the primary field which I specified when creating the entity.

This is the fields options in Legend Entries ( Series)  :

While this is the fields options in Horizontal ( Category ) Axis Labels:

There are only "Created On" and "TestName" which is the primary field.

Could anyone solve the above problem?

Thanks!