Here is an update on the refined half-over-half price changes to months of inventory (3 month moving average) relationship mohican and I optimised last year.
First the residual plots.
And the 95% CI fitted line.
We can see even with the deviation of last month we have high degree of confidence. It is easy to show a frequency component of seasonality so a high correlation is somewhat expected. Still, there ain't no purdy red dots on the bottom left and top right of the graph so it tells me a storm's a-brewin'.
And for completeness, the time plot.