Yeah, that is an interesting point and highlights the difference between "weather" and "climate" - it also brings up a tension between two different objectives of the chart: to visualize weather patterns over time intuitively, and to draw general conclusions about climate trends. In the context of the first intention, climatic oscillations like El Niño are interesting signals - you can see how they affect weather throughout the country in unexpected ways. But in the context of the latter goal, they are noise which should be filtered out/corrected for.
That makes sense; my only thought was that if you are graphing "anomalies" you might want to filter out non-anomalous behavior. Higher highs or lower lows are actually to some degree expected in those years. I suppose it could be best not to control for oscillatory behavior though as the affect of any climate shift on those oscillations is possibly not insignificant.
