First, sorry about a n00b question.
Here's the graph:
The x=30, y=0.5 is an approximation. The other two dots are measured. My goal is to mathematically describe/calculate y based on arbitrary x value.
Context
I'm regulating heating with Raspberry Pi and I have noticed that, to get increase of 1 deg C, the heating needs to work for 1/2 hours when the outside-inside temperature difference is ~10 deg. C. The measured time needed to increase room temp for 1 deg C rises to 1 hour when the in-out diff is 20 deg C. (And, as I said, 30 deg delta is approximated, for now).
The heating_speed (y axis, measured in C/h) as function of temp_delta (x axis, measured in C) just "feels" that it can not be linear. But, I could be mistaken.
Additionally, I'm not interrested in the freaky border cases when the in-out temp delta is > 40 C. In those cases, I believe, the current heating power will not be sufficient to keep the temperature stabile and it will probably have negative heating_speed.
Alternate assumption
Possibly, the function is linear, like this (blue):
In this case, I guess, the formula will be this: 3.-0.1 x.
WA link 1, link 2.
If you use what suggests Han de Bruijn, that is to say y = a Exp[-b x], since you have two data points, you can easily solve for "a" and "b". What you get is a = 4 and b = - Log[2]/10. For x = 30, you exactly get y = 0.5.