A figure from the most recent update to the Maddison Project illustrates how little we know about historical GDP.
One of my favourite graphs in recent writing on economic history might seem obscure. Reproduced below, it is found on page 28 of the working paper underlying the latest update of the Maddison Project database of historical GDP estimates. It shows the various estimates of British GDP per capita relative to US GDP per capita from 1770 to 1950. For me, this is interesting because it illustrates how little we know about levels of GDP in the past.