Dallas-based Southwest Airlines faces a federal investigation into whether it violated its own legally required customer service plan amid a blizzard of flight cancellations that ruined plans and angered travelers over the Christmas holiday.In a statement late Monday, officials at the U.S. Department of Transportation called the service meltdown, which resulted in the cancellation or delay of most of the carrier’s flights over the holiday weekend, “disproportionate and unacceptable.”As Winter Storm Elliott started to wreak havoc on a large chunk of the U.S., the vast majority of canceled flights across the nation were operated by Southwest Airlines. And air travelers’ ...