I can post later (have something to do now) some plots with M* data back to 1990-05-31, just for comparison. Recall from the other thread that both @cwags02 and I use M* data, but from different sources, and they differ slightly.
If one were to try out such a trend line for the type of analysis that Jim was doing, i.e. relating future forward return to present measure of value e.g. ‘book value’, the determination of the line can only use data in the past of the date to which you associate the future return.