How can I calculate the error between two plots?

As you see in the image, I have two plots. One of them is theoretical and the other is actual line. In actual curve, I have a shift on time axis (x-axis). I need to calculate the error between these two plot but I cannot hit the corners of that two plots. How can I fit them. The red line has 81 values and plot function compounds its points linearly. The black one has 6276 values. Plot function compounds it's points also. I should not erase or add new values to these vectors.