For anyone else also interested, I went and had a look at the links Dessalines kindly provided.

The source on the graphs says "Sources: Daniel Cox, Survey Center on American Life; Gallup Poll Social Series; FT analysis of General Social Surveys of Korea, Germany & US and the British Election Study. US data is respondent’s stated ideology. Other countries show support for liberal and conservative parties All figures are adjusted for time trend in the overall population." Where FT is financial times.

It's not clear how the words "liberal" and "conservative" were chosen, whether they're intended to mean "socially progressive" and "socially traditional" or have other connotations bound with the political parties too, and whether the original data chose those descriptions or if they're FT's inference as being "close enough" for an American audience.

Unfortunately the FT data site is refusing to let me look at them without "legitimate interest" advertising cookies so I can't tell you much more or if there's any detail on methodology.