dc dc converter - Can a system with a response Y(x) = ax + c be called linear?

I made a DC-DC converter its ideal transfer function is: $$ V_0(D) = DV_i $$

I took measurements of its response and then made a linear regression, I got this result:

$$ V_0(D) = 0.98DV_i -0.42 $$

Quite similar to the expected value. Now, can I say that the system has a linear response?

strictly speaking, the system response should be zero when the input is zero, but in this case, if D = 0:

$$ V_0 = -0.42 $$

What would be the correct way to refer at the response of this system?

Answer

Strictly speaking that is not a linear relationship between \$D\$ and \$V_o\$, it is an affine relationship. However its very rare that the distinction matters in engineering and you're likely to hear people call it "linear" anyway.