When there is a chart containing one category axis and one numerical axis the zoom functionality is supported only for the numerical axis and not the category axis (LineChartDemo1). The zoom should be available for both axes.
This isn't a bug, it was coded that way intentionally. I guess it would be possible to implement some form of "zoom" operation on a CategoryAxis...I'm not exactly sure what would be the best way (display whole categories only, or just zoom in as if the axis represented a continuous value scale and display fractional categories if that is how they fall?). Transferring to Feature Requests.