Actually I ended up writing a simple function to write a "t" anywhere on the screen by passing it x,y coordinates.
I also wrote a function to display a bell... presumably for a channel that alarms when a threshold is reached. The bell is a sub-character of the Black tank's "B".
Finally, I put a "tic" mark to the left of the graph when the bell is active showing the threshold point (at the 75% full mark on the Black tank).
If the alarm goes off, I probably could strobe the bell on and off.
Looks even better.