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.