Confidence interval of linear regression with set y-intercept

[barnabei]

Hi all, I'm looking to calculate and display the confidence interval of a linear regression with y-intercept that is inputted by the user. X is time and the value of the y-intercept will be average of the first timepoint. I've been able to create confidence intervals if the y-intercept is not constrained which give an hourglass shape but what I'm looking for is something like this:

Can anyone help? Thanks