Is Donald Trump the least presidential US president ever?

Following his most recent outbursts on Twitter, many have labelled the U.S. president as "unpresidential" and accused him of acting "beneath the dignity of the office". In the wake of this, we spoke with Professor Iwan Morgan, an expert on the American presidency from University College London, about what it means to be "presidential", and whether Trump is the least professional man to ever sit in the Oval Office.