Mbio: Volcano should be helpful when no data points are returned

[asizemore]

Needs discussion.

Noticed when using a test dataset (called "test) that has only 10 samples.

I think there is a lower limit on how many samples the differential abundance methods need in order to return actual results. So this combination returned ids - so the compute didn't fail - but no statistics. I'm 99% sure this is because there are so few samples in the selection. I can select more bins for GroupA and B and then i do get results.

Anyways, instead of a blank plot, the plot should have some words like "No data returned" or something similar.