Referring to the chart:
What do the units represent? Is it directly related to GNP?
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Equities is another word for stocks. It looks like they are graphing against some kind of index. I checked the Wall Street Crash of 1929 on Wikipedia, but that uses a different scale for the Dow Jones Industrial Average, which was my first thought. They may be using a scale normalized to the value on January 2nd, 1929 or similar. A value of 100 at the beginning would suggest that.
Equities are not directly related to GNP or GDP. They can go down when GNP is going up. For example, the United States today or in 1986.
P.S. Smoot and Hawley would not have lost their seats in midterm elections in 1932. That was a presidential year. The closest midterm elections were in 1930 and 1934. A little research suggests that both lost in the presidential election of 1932, Hawley in the primary and Smoot in the general. The arrow may be in the right place for Hawley but not Smoot.
As mentioned in Brythan's answer, equities is another word for stock.
Without knowing the source of the article, it is impossible to know what was used in this graph to represent the 'US Equities' curve.
It could be the Dow Jones or some aggregate index computed in a different way. It is likely that the author used an index like the Dow Jones as a proxy for the valuation of all US stocks.
For convenience, whatever was used to represent the US equities has been normalized at 100 at the beginning of the period in 1929, so you can read the chart as a percentage evolution over time.
As per Wikipedia, Gross national product (GNP) is the market value of all the goods and services produced in one year by labor and property supplied by the citizens of a country. GNP
US Equity represents the valuation of the companies in the US. This does not have a single definition but the most common one would be, for each company, number of stocks issued for this company multiplied by the market value of a single stock (or market capitalization of a company).
So GNP and US Equities are not directly related, i.e. the GNP can increase while stock valuation can go down.