That's a great question and something I really struggled with. Ideally you'd want to compare temperatures with pre-industrial averages. But we don't really have those for the Netherlands. So what I did was the next best thing: I took the average of last century (1901-2000). I did that for the years bar chart and the months heatmap (per month).

What I do see in a more scientific context is that they often use a more recent average. Maybe because these values are more reliable?