Tidying up today I found a photocopy of a photocopy of the above map.
Beginning on the left at the Polish-Russian border, the grey band shows the size of the army as it invaded Russia in June 1812. It started with 422, 000 men and finally reached Moscow with 100, 000 men. The black band shows the army’s retreat from Moscow, finally making it back to Poland with 10,000 men. It is this black band that is linked to the temperatures and dates at the bottom of the chart. Altogether there are six variables plotted - can you see them all?

There is a large and detailed version of this map on the Napoleonic Literature website. Some interesting versions of this map can also be found here.