You might be wondering who invented the pie chart? Well it was not Florence Nightingale (she published for the first time different types of charts). The Pie chart was invented by William Playfair and was first published in 1801.
The above example is included in "The visual display of quantity information" written by Edward R. Tufte.
Florence Nightingale pie chart
What about Florence Nightingale? What did she used her pie charts for?
Here is her example taken from wikipedia.
She used it to support her objective by presenting the causes of mortality in the army in the east and I'm guessing presenting a solution? I like her example because it focuses on very specific message and period of time and it does it in a very interesting and clear way.
The graph is supported by commentary and interestingly you cannot read the graph without it, so you look at the "pie chart" notice blue wedge and Florence start in second paragraph from explaining most obvious blue wedge (instead of red) which to me gives a clear message as it very easy to see the key problem.