At 6 inches the data was missing, which is why the graph appears to drop off. The register key is at 4.5 inches, 1st tone hole at 5.75, 2nd at 6.25, and 3rd at 7.5. At all of these points it can be seen that the odd harmonics are all nodes, while the even harmonics are antinodes. The missing data point would help illuminate what is happening in that important area right between two different tone holes, but a clear enough picture can be obtained looking at the slopes on both sides. Here only the 2nd harmonic exhibits the same behavior as the notes before this one.