How do I force a Linear fit to go through a particular Y value at X = 0?

To force a linear fit to go through a particular Y value at X = 0 you have to use the General fit. For example if you wanted to have Y=4 at X=0 you would use the equation:

4 + b * x;

in the General fit dialog. Similarly for Y=0 @ X=0 (the origin) you would use 0 + b * x;