Nightingale explains her diagram

Nightingale published her famous diagram in 1859, in A Contribution to The Sanitary History of the British Army. On page 9 of the book ( see below) she explained that she designed the diagram to show how we could reduce deaths from  infectious disease by public health measures, particularly the drainage of homes. This applied to civilian towns rather than just hospitals and army barracks. 
It was a ‘before and after’ diagram showing evidence of the potential benefits of “improvement” of towns, meaning above all installing home drainage. She says that coloured diagram No. 1 (on the right, showing the higher level of mortality) would apply to towns in an “unimproved” state. No. 2 (left) showed what the mortality would be if the town was “improved” by public health measures. This was the start of her campaign for public health legislation, based on the terrible lessons of the Crimean War.
Page 9 of A Contribution to the Sanitary History of the British Army (1859)

You can see the full text of the book online at Harvard University.