I think DOT only tracks on a by-flight basis ... the airlines aggregate as they deem most useful for marketing purposes

for a publication of WSJ's reputation, that article contains some pretty simplistic statements:
"Many passengers say they want the quickest trip possible."
"American tries to put planes with lots of connecting passengers at nearby gates to cut down on long walks ..."
"If bad weather hits at the wrong time, diverted flights and missed connections can cause widespread delays."
"Getting bags onto their next flight is a challenge with shorter connections."