Nitpick: it's not a graph of "global temperature", it's a graph of temperature anomaly. The anomaly for a single site is the difference between the current temp

Nitpick: it's not a graph of "global temperature", it's a graph of temperature anomaly. The anomaly for a single site is the difference between the current temperature and the average for a fixed period in the past for that same site. Temperature would instead be the actual value of temperature. Calculating the average global temperature is rather more difficult (https://www.ncdc.noaa.gov/monitoring-references/faq/anomalies.php https://www.ncdc.noaa.gov/monitoring-references/faq/anomalie...). See https://earthobservatory.nasa.gov/Features/WorldOfChange/decadaltemp.php https://earthobservatory.nasa.gov/Features/WorldOfChange/dec... which I think also addresses your other questions.