Data is, in essence, a lot of numbers. They mean something only when put together and analyzed. However, the analysis can be lost on someone who is not intimately familiar with the data and/or how to read the analysis. It is here that data visualization helps.
I came across this interesting post on tableau.com about data visualization at the turn of the century: How W.E.B. Du Bois used data visualization to confront prejudice in the early 20th century
I believe it goes to show the power of data visualization and how it can be used to show uncomfortable truths in a simple and straightforward manner that is still very effective.