I think I'd prefer this to be a more general `sortby` argument, where the `sortby` can be any of the `measures` or `sample_variables`. There is analogous sorting support in the `plot_heatmap` function for sorting either the OTU or sample indices (or both) by a specified variable. In this way, `"Observed"` would just be one possible argument of many. If more general I think this argument would be worth supporting and I would merge. 

Thanks! And let me know if you're willing to mock-up the more general case. I don't want to merge as-is because I don't want to temporarily add an argument I'm soon going to replace with something else...