Look at your sampling time of your constant. By default, it is inf. This means it is sampled once at 0, and never again.
Change your sample time to what you want it to be to get a line.
However, this may not be necessary. A constant is just that, constant. If you add a time-dependent signal (sine wave, step) to the second port of your scope and do not change your constant sample time, you will see the constant value is present every time the time-varying signal is sampled.
