Learn English – Difference between “undertone” and “overtone”

In general, if you have a number as an adjective, such as five percent of cases go unsolved, 76 percent of them voted in favor, or 50 percent of coin throws land on heads, you use percent of.

If you have roughly the same percentage of men and women, a low percentage chance of failure, or a policy meant to decrease the percentage of drivers without insurance from 25% to 15%, percentage of is more common.

So, in your example, I personally would use frequency as a percentage of the total in a sentence. The other would be perfectly understandable too, and I may favor it (not sure though) as a graph title.

The difference is largely one of connotation. The verb to shroud derives from the noun shroud, which typically refers to a sheet used to cover the dead for burial in some religious traditions. Because of this association, when you say that something is shrouded with or shrouded in, it connotes an atmosphere of mystery, gloom, or the numinous.

Shrouded in also lends itself more readily to metaphorical usages. You might say that "The castle was shrouded in mist", but it would sound odd to say that "The castle was covered in mist".