What did Francis Galton produce in 1881 that demonstrated the public appeal of mapping intensity using color gradients, although it was not a true matrix heatmap?

Answer

An early isochrone map showing travel times

Francis Galton's 1881 visualization mapped travel durations from London to various global locations, utilizing distinct colors for different time bands, showcasing the efficacy of color intensity mapping for public appeal.

What did Francis Galton produce in 1881 that demonstrated the public appeal of mapping intensity using color gradients, although it was not a true matrix heatmap?
inventorvisualizationheat map