Roy, good questions. Yes it's actual thrust data from the Columbia mission, STS-107. However you can't directly measure thrust. Rather it's calculated from other parameters such as chamber pressure. Also the thrust curve is generally corrected to constant sea level or constant vacuum thrust. In actuality the physical thrust would be constantly changing, even if engine thrust was constant (which it's not for the SRBs). I include below some more info I was preparing to post:

Because of the frequent confusion on this point, I uploaded a thrust/time graph for the SRBs. I also revised the thrust numbers in the article to reflect this. SRB sea level liftoff thrust is 2.8 million pounds each. Each SSME produces 393,000 pounds sea level thrust at 104% power level. Thus the shuttle total liftoff thrust is: (393,000 * 3) + (2,800,000 * 2) = 6.779 million pounds force, or 3.075 million kgf, or 30.16 MN. Using these numbers the SRBs provide 82.6% of liftoff thrust. 

This is confusing because many sources just blindly copy the 3.3 million pound thrust number, some even erroneously adding "sea level" or "liftoff" thrust. There are six possible thrust numbers for the SRB:

 Sea level liftoff thrust
 Sea level peak thrust
 Sea level average thrust
 Vacuum liftoff thrust
 Vacuum peak thrust
 Vacuum average thrust

The problem is most references don't state which they are using. This is why references state SRB thrust from 2.65 million lbs (probably sea level average thrust) to 3.4 million lbs (probably vacuum peak thrust).

When comparing SRB thrust to, say, Saturn V F-1 engine thrust, you must consider the different characteristics of each engine type. The F-1 is not throttlable, so excluding altitude variations always produces the same thrust. At sea level this is 1.5298 million lbs per engine or 7.649 million lbs thrust total. By comparision the SRB thrust changes over the burn time (see chart). Space shuttle total sea level liftoff thrust is 6.779 million pounds force, or about 88.6% of the Saturn V.

In theory you could compare the SRB thrust at T+20 sec which is somewhat higher, but that wouldn't be liftoff thrust.