When you choose to create a new panel, one of the options should be "Discrete". I believe it is a built in panel...if it's not there just search for it under Configuration (cog icon) -> Plugins and install it.
Before I think about trying to implement something like this would like to understand what level of knowledge or experience Is required to implement this. I really like what I see but more times than not it seems it decent level of programming is required for any of this or a deeper level of understanding of Linux.
What sensor do you use to monitor the hot tub temperature?
The one built in to my Intermatic heater.