Automatic linear curve??

I have a process which uses a linear curve. I have been using the FGEN block in RS5000 for this, and it works great. The problem is the machine Im controlling changes the curve betweek Summer and Winter mode. So the curve is off by a little bit depending on the time of year. Just wondering if someone has a better solution for a automatic linear curve.
Thanks for you suggestions.

X axis = Units
Y axis = 0-100% for the controlling Valve

If you only have two curves (one for summer one for winter), switching between two FGEN will do.

BTW, I like the FGEN, in the most case, if you have enough points and memory, you can get any curves you want...

Already tried switching the two - But I shouldve said it changes more from winter to summer - but also day to day. Its never the same on any two days. PID wont work because it needs to be a really slow process - which I created a RAMP AOI to ramp up to SP really slow.

What is a linear curve besides being an oxymoron?

Inquiring minds want to know.

True - not very - thanks