TruncatedSeq::predict_next() attempts linear regression with only one data point, this leads to a division by zero. (There are infinit many linear functions that fit equally well for a single point).

I suggest we do what we do for the zero points case, namely pick one of the linear functions.

For 0 points the current version picks `y = 0 + 0*x` and the suggestion is that for one point `P` the function `y = P_y + 0*x` is picked.