There's enough women in the profession to have female -> female lines originating out of them, though. The implication is that there's a huge majority of queer women in them, to the point where heterosexual ones won't even show up on a visualization despite how much of a minority GLBT people are in the general population. (Or, alternatively: heterosexual women in those professions don't marry for whatever reason, and queer ones do.)
Same-sex occupation/relationship matchups weren’t common enough to reach the top five in any occupation.
So the chart also highlights the top male-male and female-female job matchups for each occupation.
In other words, female -> female pairings are overemphasized in the chart, otherwise you would not see them at all.l
No, the visualization is adjusted such that it always shows at least one female->female connection, and one male->male connection.
So it might very well be the case that there are a lot more heterosexual female welders than homosexual female welders, it's just that there are so many male welders that their connections dominate the top five.