I think there's actually not 2 bars per year, but instead:

• the first red bar = the year 2000
• the second bar (which is not paired with another bar, and also, is not labelled at all) = the year 2002
• the third bar = the year 2004
• the fourth bar = the year 2006
• and so on until the final bar, the twelfth one, = the year 2022

The x axis increases by 2 years each entry.

The number after the "|" is only meant to "helpfully" (and confusingly) tell you when those children were born. To take your example:

• the data for the year 2000 is represented by the first red bar (7 per 1000 eight year olds had autism in 2000) and is labelled as 2000 | 1992 because those kids were born in 1992.
• the data for the year 2008 is represented by the fifth red bar (11 per 1000 eight year olds had autism in 2008) and is labelled as 2008 | 2000 because those kids were born in 2000.

When they collected the data in 2000, about seven 8 year olds out of 1000 had autism. Those children were born in 1992.

When they collected the data in 2002, about six 8 year olds out of 1000 had autism. Those children were born in 1994. This information was not labelled on the x axis.

When they collected the data in 2004, about eight 8 year olds out of 1000 had autism. Those children were born in 1996.

When they collected the data in 2006, about nine 8 year olds out of 1000 had autism. Those children were born in 1998. This information was not labelled on the x axis.

When they collected the data in 2008, about eleven 8 year olds out of 1000 had autism. Those children were born in 2000.

... I'm too lazy to continue but ...

When they collected the data in 2020, about twenty seven 8 year olds out of 1000 had autism. Those children were born in 2012.

When they collected the data in 2022, about thirty two 8 year olds out of 1000 had autism. Those children were born in 2014. This information was not labelled on the x axis.

I'm not positive i'm reading it right, but that's what I think they were trying to convey with this (terribly labeled) graph.

eta: yeah, rereading the subtitle, those numbers and years make sense