Looks like the hourly temperature plot graphs tend to get darker / "denser" as you look from left to right. Is that more data being available, more fluctuation

Looks like the hourly temperature plot graphs tend to get darker / "denser" as you look from left to right. Is that more data being available, more fluctuation, less fluctuation? What is that? I can also see some trends like the variation increasing. What's different when using another temperature data source? It would be nicer if the R source was just public without having to ask for it. The author is clearly a knowledgeable person, they still might benefit from asking themselves what they don't know before drawing any strong conclusions.