The plot references are always the center of the plot area. That grid line has > and < markers on ends.
Plot scale factors and the reference values can either be fixed values or auto-scaled / auto centered.
Heather calculates various statistics values (RMS, AVG, SDEV, VAR, MIN, MAX, SPAN, etc) for each plot. The value shown on the screen is for the portion of the plot queue data shown in the plot area. You can select which statistic you want to see.
All this is documented in the .pdf file or the comments at the start of the heather.cpp file... please read it.
Also, it looks like you changed the color map for the temperature value... it is normally YELLOW.