I(R1) normally, then move the mouse cursor over the x-axis, until it turns into a ruler, right-click on it, then write
v(2) instead of
time. To get back to
time, simply delete whatever expression there is instead of
time and click OK.
Judging by the frequency and topics of the questions you posted, I'd rather suggest you to try and read the manual. The LTwiki might complement with some little hidden treasures. But it's best if you try to get to know the tool with which you're about to spend time with, rather than stop at every step and wait for answers online. Nobody says you have to know it perfectly, but enough to know how to work with it, without getting frustrated that you cannot continue because of something trivial.