You misread the chart slightly. ... The top line shows total debt ...
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No, the top line of this graph shows the Total Credit Market Debt as a Percentage of GDP. For 1933 it shows a peak that is labeled “270%,” and that is in agreement with the scales.
If this is not true, but the top line is supposed to show the total debt - as you write - well, then the graph is, as I suspected, not correct.