How do I fit two linear curve fit segments that meet at a breakpoint?

Use a formula like: ((m0 < m1) ? m2+m3*m0 : 0) + ((m0 >= m1) ? m3*m1 + m2+m4*(m0-m1) : 0)

m1 is the breakpoint, m2 is the intercept of the first segment, m3 is the slope of the first segment, and m4 is the slope of the second segment

You may want to lower the Allowable Error to 0.1 or less to make sure you get a good value for the breakpoint.