[amibroker] Plot current day's middle price

[dralexchambers]

I would like to plot the current day's middle price, starting from 
13:00 each day. So at 13:00, the indicator "starts" and plots:

(current high value since 13:00 + current low value since 13:00) / 2

I have intraday data on a 1min timescale.

Thanks!