It took me a few looks to figure this out, but out of all the various y axes on this chart, this appears to correspond to "business volume scale," which is exac

It took me a few looks to figure this out, but out of all the various y axes on this chart, this appears to correspond to "business volume scale," which is exactly 0 on the flat line. The axis itself is found under the years 1845, 1895, and 1930. What "business volume scale" means or how it is measured, however, I don't see specified anywhere, and after an admittedly cursory web search plus FRASER search of 20 minutes or so, I can't figure it out. The text blurb for "business activity" seems to mean it is probably some aggregation of industrial production and consumer spending, but what 0 is indexed to and what the deviations mean is less clear.