The rally and retreat behavior is a good feature because it helps to establish a second anchor point for
a new trend line.
The slope of
this new trend line is different from the slope of the long term uptrend line.
A new trend line would begin.
Past NAEP practice has been to start
a new trend line when a new framework is introduced.
It is possible that some of the trends will turn out to be too low, because of the possibility of switching to
a new trend line, as the British did after the war.
There may be a move to
a new trend line underway.
To start
a new trend line from the 1998 temperature point breaks the continuity of the trend.