Alph2Z, it seems to me your graph is off.
As the green line (speed) is a function of the red line (deceleration), it should become less steep after 5000' as deceleration becomes less.
The blue and purple dots at 0' for other aircraft are completely meaningless on a graph like this.
And the deceleration at 6000' shows -1.5 obviously, not -1.8 as you write. That would be at 5000'.