Well, as you probably know, lines are normally drawn by indicators. And their lines are actually a little bit different kind of lines. If you try the "Trace" block, it draws a line, but that line is made out of multiple trend-lines, multiple objects. Compared to that, Moving Average draws a line that does not appear on the list of objects. So the best is to make a custom indicator. EAs are primarily designed to read indicators data and put trades and orders here and there.