Did you put it on the Y-axis instead of X-axis? To put a reference line on the X-axis, you need to select "continuous" for the dimension. Use the number tab to fix the formatting. An X-axis reference line defined with either of the expressions above then works just fine for me.
Chart_Time isn't a time, it's a timestamp. You don't display the date, but it's still there. If you want to keep the date, I highly recommend displaying it to avoid the exact kind of confusion you just went through. However, then asking for a reference line at 7:00 AM makes no sense to me because on which DATE do you want that line? All of them? The first one?
If you don't want the date in your time, then strip it out using something like frac(). Then if you use a continuous axis, something like the suggested 7/24 or your time('07 AM','hh TT') should work fine.