I think that non-linear effects are expected because:
the gas demand declines linearly
the green gas share increases linearly
the amount of green has is the product of the two and a product of two linear effects gives quadratic terms.
Having said that, it might be good to make a simple Excel calculation with the correct start and end values for the total amount of gas and the green gas fraction and check if you get the same curves.
redekok
commented
5 years ago 
As suggested here I made an issue
I think that non-linear effects are expected because:
Having said that, it might be good to make a simple Excel calculation with the correct start and end values for the total amount of gas and the green gas fraction and check if you get the same curves.