Google-sheets – Define a range for a plot that contains every fifth row of data

Glad it worked! Copying over the answer:

You can try something like this: =J4 + ArrayFormula(sum(if(mod(row(J5:J)-4,9)=0,J5:J)))

I've modified the code from my answer to fit your description. In the example below, columns 2-5 (that is, B-E) of sheet named 'Sheet1' are monitored for changes. In case of change, a timestamp is inserted in the last row of that column.

To customize: change the sheetname and column bounds on line 4 of the script. If a non-contiguous range of columns is to be watched (e.g., columns 2,4,7,8), replace the conditions column >= 2 && column <= 5 with, for example, [2, 4, 7, 8].indexOf(column) > -1

function onEdit(e) {
  var sheet = e.range.getSheet();
  var column = e.range.getColumn();
  if (sheet.getSheetName() == 'Sheet1' && column >= 2 && column <= 5) {
    var row = sheet.getMaxRows();
    sheet.getRange(row, column).setValue(new Date());
  }
}