I just added a curve for my MGTC - CSI-Pro (Curve 11) that maxes at 40 degrees at 4000 RPM. When I compare this graph the line extends beyond 40 degrees as RPM advances beyond 4000 RPM.
I tried adding more points beyond 4000 RPM but maintaining 40 degrees but the graph stills extends beyond 40 degrees. Is this expected behaviour for the graphing software?
The answer to your question as to why "line extends beyond 40 degrees as RPM advances beyond 4000 RPM" is that YES it is due to the way the graphing software works.
In practice, the advance needed by an engine follows a smooth curve. As it is very difficult to get a mechanical distributor to produce a smooth curve, the Lucas distributors produced a 3-step approximation. This basically consists of 3 straight lines with decreasing gradient as the engine RPM increases.
Very approximately, from 0 to 1500RPM, a weak spring allows the bob weights to fly out quite quickly giving a rapid rate of advance. At 1500RPM, a stronger spring engages slowing the rate of advance. Finally around 3000 the bob weights hit a stop that prevents any further advance. In practice, these 3-step advance curves provide a reasonable approximation.
With modern cars or with programmable electronic ignition systems, it is possible to program in a smooth advance curve. (Or use the knock sensor to determine the best advance).
To answer your question. The graphing software fits a smooth curve to the points you provide to give a "best guess" at the ideal curve. There are 3 choices of curves to help you choose the one that gives the best fit.
Your curve appears to give an excellent fit and yes, I would expect the "real" advance required by the engine to continue to increase beyond 4000RPM.
All the best