The above link is not working. However I found this Add Comparison Type: % of row (In Discover) in the ideas section which goes into the same direction and voted for it. This is a much required feature in our company. We almost daily face this restriction in our analysis
Downloading in a csv defeats the purpose of analytics workspace. Hopefully the Adobe guys can add this is as it's a pretty straight forward feature. I really don't know why the default is by column, most people would be doing trended tables and the % of the column is useless.
In my knowledge there is no option of breaking down by row. It makes sense in your case as you have a single metric in the column (visits). But if you have more than one metric in a table (like most of workspace projects) then row-wise percentages would become invalid. One thing you can do is download it as csv and calculate the percentages in excel.