There is a plugin app in ThingSpeak that you could use to crate a javascript app with a dropdown menu (I suspect, though I havent tried) However, you cannot make plugins public, so you would only be able to use it yourself. But if you do make one, you can embed it on a private view of your channel.
I would also suggest using the charts API, if gives you easy access to charting your data, and you could create a seperate site that calls the charts API to show the desied data. I've sen some sucess using google sites to generate such a thing perhaps. The easiest way to do this would be to create a MATLAB app in appdesigner if you have a MATLAB license.
Actually the javascript plugin worked really well! This took me about 2 minutes to make, using this link. One problem is that the button dindt persist, I think you need to write the html so the button stays and then link opens in a subpanel or somemthing. Bet its possible with just a bit more effort.