@ehBeth,
Underlining something, and drawing a line under something have the same meaning, so far as I know. My point was that 'draw a line' by itself is usually used in the sense of drawing a line in the sand, or maybe establishing a deadline. In otherwords, establishing a line that can't be crossed. When I first saw the words draw a line (under) something, I was momentarily diverted from the actual meaning. My mind sometimes goes in odd directions. I should really save it for something important.
We also use the underline to mark the end of a column of numbers, as you know. Put all the revenues in a column and underline the last number. Do the same with expenses. When you find the difference, double underline the net income or loss. All done. Finished.