The chart shows the Adjusted Nakamoto Coefficient, which for Nano is the minimum number of representatives that together form a majority. Adjusted means that representatives controlled by the same entity are grouped together (see groups). For example, this means that all official representatives are grouped together, their weights are summed up and the sum is seen as one entity.
All includes all representatives and Online only includes online representatives (the supply is then the total online weight at that time).
The chart shows the Unadjusted Nakamoto Coefficient, where no representatives are grouped together.