Can't find a graph with those mysterious terrorist threats (RED!
YELLOW! ORANGE!)listed by date of when pronounced, duration of threat,
whatever. I can't think of a way of measuring so called threats prior
to the establishment of the color system a few years ago.... Someone
must have made a graph/chart!

Homeland Security, the department responsible for issuing the stages
has a place on their web site where they list the annoucements from
March 2003 to current. The hard part about charting it is that there
is no overall threat level for the entire country as you will see. The
levels differ by region or, as of this moment, business sigments such
as financial institutions in Newark, NJ and New York City.

If you want to make an assessment of prior threat levels in relation
to the current table you could use the DHS description of what
consitutes each level. Not scientific but really all you have to work
with.

You might also find it helpful that ABC News was producing a "The
Insider: Daily Terrorism Report" for tracking this information.

DHS Threat Advisories, Bulletins, Memoranda
http://www.dhs.gov/dhspublic/interapp/editorial/editorial_0335.xml
DHS description so of each threat level
http://www.dhs.gov/dhspublic/interapp/press_release/press_release_0046.xml