"but when I repeat the process with a constant irradiance, and Temperature as the input variable, I do not get the desirable result."
Temperature is already constant
T = 298.15;
T_0 = 298.15;
With constant irradiance, you will get the same results, I_r pass the constant I_r vaure for individual plot.,,same thing
Two plots with two plot commnad, please check, now When
As changing the I_r, y scale changes to fit the plot,
Therefore when you tried with
You will get 10 plots, with constant I_r value (200,400,600,800,1000), just y axes scaling is changing to fit the ranges of all plots.
You can verify the same , by passing single contant I_r value
for I_r = 200:200:1000