Moral Statistics and the Thematic Maps of Joseph Fletcher
Maps, which date back at least 4,000 years, are the oldest of graphic displays. This isn’t surprising, for it is a natural metaphor to represent space as space. But adding a third variable onto a geographic background is a more recent innovation. Of course, there were partially thematic maps (in China) as early as 164 BC, but the additional variables plotted were geographic variables such as altitude, which were ancillary to the main purpose of the map. A thematic map whose focus was primarily on the other variable did not appear until the 17th century.
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