Again - it's not arbitrary! It's chosen because industrial emissions have affected the climate. There is a very clear reason why it was chosen. That's why we use the word anomaly too - it isn't chance!

Are you trying to say that it's not an anomaly at the beginning of the period in the chart when the delta is negative? I already granted that point but it doesn't matter because everybody already knows there is an actual anomaly due to industrial emissions! The "implication" that the 10 years were "normal" is not questionable because the temperature was normal before! This case doesn't need to be made here because it's been made by a global coalition of governments and scientists in exhaustive detail.

From Wikipedia on mini ice age:

"...a multi-centennial period of relatively low temperature beginning around the 15th century, with GMST averaging –0.03 [–0.30 to 0.06] °C between 1450 and 1850 relative to 1850–1900."

This is within the annual variation I was suggesting and is not comparable to what we see today. It's irrelevant.