[E. Pluribus Unum]

The blue label says “Temperature change from 1950 to 1980.” But each line on the chart represents 10,000 years.

It actually says "Temperature Change 1950-1980 Mean."

They are using the mean (average) temperature during that period as the reference temperature. In other words, the period 1950-1980 is "0.0" on the Y-axis. The temperatures for all other years are degrees above or degrees below the average temperature for the 1950-1980 period.

There is no obfuscation going on here. They are perfectly reasonable scaling factors.

[Excellence]

Ah, thanks for straightening that out.
My first thought is that choosing that time span as a reference point is rather arbitrary. What says this particular reference point is “normal” and everything outside of it is “too” hot or cold?