I've come across this chart from a few months ago and am unable to trace down its origins. But I would like to think I should be able to decipher it even without the authors whispering in my ear. So even without the original text, I want to see what important conclusions can be drawn from the chart:
I have a cursory understanding of the x and y axes. All the data is available from St. Louis Fed (only tricky thing is that the temporal component is not shown, aside from "you are here" denoting time of writing). Also, IOER seems to have been replaced with IORB since. Methodology aside, I struggle to grasp what t-bill/IOER spreads are supposed to reflect. I'm guessing it is some kind of anomaly if risk free rate is not proportional to money supply growth, but I don't understand the intuition.
Clearly, the authors of the chart were keen to flag falling spreads in a highly liquid environment, but is there an easy explanation for what implications are hinging on such a relationship?