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?
In this case, it seems clear. Because the column is telling about one store and its visits (of customers). Let's take that as an example:
"How many visits of customers do you notice in a year for this store?"
over...
"There was just one visit in a year for this store."
Since the rows of visit column is likely to have more than one visit, it is labeled as visits. On the other hand, the first column's all rows will have only one store and thus it has the heading of store.
In most of such cases, we need to think what exactly the row will define. For instance, a table on the very first page of a textbook may contain
Index (the lesson number - singular)
Lesson (the title of ONE lesson - singular)
Page number (one page number of that lesson - singular).
On the other hand, we will have heading in plural in some cases where the row reflects plural matter.
Say we are having a price list of some items.
Here, the second column takes Item as a singular because, I'm defining one item per row.
Now we have a grade list of furniture
Certainly, the second column shall have the word Items plural because each row is defining the items that are more than one.
