How To Label a Vertical Grid Line?

columns and rows have their own labels: 1,2,3… A,B,C etc.
And we can put our own names on those rows or those columns.
All that is fine.
But I want to name the line. The actual vertical grid line.
Because my spreadsheet is an index of maps and the number of the map that covers the area between, say, N50° and N50° 20’ on one side and E22° and E22° 30’ along the top fills that cell and the boundaries of the cell are the longtitude and latitudes.
To put it perhaps a simpler way: the grid lines on the spreadsheet represent lattitude and longtitude coordinates so it them I want to label, not the spaces between them.
Is there any way to do this?