I am seeing the same thing on an IAP-225 running v.4.1.0.1.
You should also get that information if you click on the 2.4 / 5GHz links in the upper-right corner of the spectrum dashboard then hover over the bars in the graph; however, I am not seeing that either. I tried on Firfox and Chrome.
Please open a case with TAC to get a bug ID to track it to a fix. Please keep us posted.