Fit a linear model and get its R^2 if I want the model to have a specific form and slope

I want to see fit a model y=mx+c when m=1 to some data, so I only want to learn the offset.

I know the expressions are relatively easy to get by hand but I was curious if some software in matlab already did this, especially if it computed other stuff for me like R^2 values etc.

Does this exist?

I have two vectors X,y and want to get y=x+c but only fit c.

[KSSV]

Have a look on regression it will give you R.