> Of course, we could cut some corners here. She could download the data as a CSV file, load it into Excel, define a couple of new columns to hold the Kelvin equivalents of the Celsius temperatures, and then plot a graph

Why does she need to convert to Kelvin? She's trying to compare differences in average temperatures. Wouldn't that be the same in Celsius?

>the gap between the average annual temperatures

She is probably using the percent difference, where an absolute zero scale should be used.