Here is the correct answer from Tableau Support.
My name is Jon and I will be assisting you on this case. I see that you have noticed that one of the trend lines (series A) stopped when it hits roughly the 80 million mark on the Y-Axis.
This is occurring because Tableau Desktop stops drawing the trend line at a point determined by the distance between the highest and lowest point on the X and Y axis. We do this because computationally we are drawing a vector that could, hypothetically, continue forever and use up system resources. We can see this behavior in action by selecting any two points on the line.
I will enter an enhancement request on your behalf for the Product Management team to review for possible inclusion in a future release. Another option to provide suggestions for product enhancements is to submit an idea through our community: http://community.tableau.com/community/ideas Providing feedback through the community forums allows other users to vote and discuss enhancement requests, and allows our development team to gauge the demand for each enhancement. Your feedback is invaluable and helps us improve our software.
Since the enhancement request has been submitted on your behalf, I will be moving this case into a closed status. If there is anything else we can do to help please let me know by replying to this email and this case will automatically be marked as active.