It's a map, not a graph so there is no y-axis. The colored area represents the size of the army with the gold area being March to Moscow and the black being the retreat. There were two small groups that split from the main thrust. The black are them rejoining the retreat. As you can see you were luck if you were in one of those two groups since they did not get decimated.

See http://www.masswerk.at/minard for a translation of the map's original legend (mouseover). There wasn't just a single movement of the army, but several troops were detached or left behind and rejoined later in the campaign. There's also a bit of a simplification in the map, as was pointed out by Minard, in order to provide the bigger picture of the events in the flow chart.

You can mouse over a point on either the map or the graph to highlight the corresponding point on the other. Based on that, the spike in late November appears to represent the main army rejoining a group that split off during the outbound march.

As you can see in this graph, Napoleon split up his army and left a small part behind while he walked towards Moscow. It's the remainings from this smaller army that represents this spike