Basically, most of Trump's nominees are still awaiting to clear the respective Senate committees.
As seen from The Washington Post graphic that you cited, most of the nominees are only referred to the respective Senate committee in May, which is just a month ago.
Nominations must be formally submitted to the relevant Senate committee before a vote in the Senate can take place. Thus, by just announcing nominees and not formally nominating them, they cannot be confirmed by the Senate.
As this article by Politico mentions, the administration didn't file the necessary paperwork for many of the nominees to make the nominations formal.
President Donald Trump is lashing out at Democrats for allegedly stalling his appointments and agenda, but it’s his own administration that is frequently sitting on the necessary paperwork for nominees.
However, there are also other reasons for the delay. Firstly, it may be delayed by the Office of Government Ethics as they can't review the nominees' financial reports until the administration submits them. The administration has been slow to send the nominees' financial information to the OGE, according to an OGE spokesman.
South Dakota Sen. John Thune, the third-ranking Senate Republican, said some nominations may have been bottlenecked at the Office of Government Ethics, which helps hash out ethics agreements for government appointees.
But an OGE spokesman suggested the White House has been slow to send them nominees’ financial information. “OGE can't review reports until we receive them,” a spokesman said. “Once we have received them, OGE has been moving these reports faster than we did in the 2009 transition.”
Also, even though Democrats cannot reject nominees as they do not have the votes to do so, they are able to delay the nomination process, as mentioned in the same article:
Once the nominations hit the Senate, many face political hurdles. Although Democrats can no longer defeat nominees through the filibuster, they are still pulling all the procedural levers they can to extract key concessions from the administration. One example is the nomination of Sigal Mandelker, a top Treasury Department official, which Democrats have held up to try to obtain documents involving Russia’s financial dealings with Trump associates.