singular - Can I use plural nouns in table headings?

I've always thought that table headings need to be singular. However, I've come across examples when that hasn't been the case.

For example, if there is a table that shows the number of visits that each store had in a year from all over the country, the heading (the first column) that lists stores would be "Store," but the heading (the second column) that lists the number of visits would be "Visits" instead of "Visit."

What would be the rule to follow?