What it shows
The image of the outbreak, shows that there are an increase of deaths around the Broad Street pumps which could show that the water is dirty in the Broad Street Pump ...
It was later revealed, that the cesspit in Broad Street which was 1 metre away from the pump and had a crack in the side, so was polluting the clean water with the cesspit contents!
What he did
John Snow removed the tap from the pump and .... the epidemic ended!