May be Epilog?

Plot[x, {x, -1, 2}, Epilog -> Line[{{1, -1}, {1, 3}}]]

gives

May be Epilog?

Plot[x, {x, -1, 2}, Epilog -> Line[{{1, -1}, {1, 3}}]]

gives

You can change the style as needed. For example

Plot[x, {x, -1, 2}, Epilog -> {Dashed, Red, Line[{{1, -1}, {1, 3}}]}]