Just eyeballing the "days of supply
" chart, and then eyeballing the "gasoline imports
" chart, it looks like there is a relationship between diminishing "days of supply" and increasing "gasoline imports."
I'm just speculating, but a barrel of domestically refined gasoline likely contributes more to the "days of supply" count than a barrel of imported gasoline, since many of the domestic barrel will spend some time traveling through long pipelines to get to markets on the east and west coasts. On the other hand, much of the imported gasoline likely arrives at ports nearer coastal population centers and doesn't spend must time contributing to days of supply. As we import a higher percentage of our gasoline consumption, the days of supply count will naturally drop.
So I don't think that a low "days of supply" necessarily implies that pipelines are dropping toward the minimum operating oil.