Warning. The blood-red lines on the first global disease maps, printed in 1832 medical treatises, caused alarm among readers. The maps were intended to inform the public and help stop the spread of the disease, but those who thought a fear of cholera predisposed people to contract it worried that viewing the maps itself posed a risk to their health.
Erasure. What a data visualization leaves out or obscures is as important as what it shows. Plantation ledgers, following the advice of Thomas Affleck's 1847 "Cotton Plantation Record and Account Book" presented grids of numbers that look "like antiquarian versions of Excel spreadsheets." These charts made productivity visible but made the brutality of slavery harder to see.
Problem-solving. Customs appraiser Haydn M. Baker chemically tested imported wool to determine its value. In 1886, he tried to "derive, geometrically, the 'theoretic separation' between the categories 'washed,' 'unwashed,' and 'scoured.'" The complicated chart he produced demonstrates the difficulty of visualizing uncertainty.
Reform. College-educated women ran Hull House, providing services to alleviate poverty and aid immigrants' assimilation into American life. Led by Agnes Sinclair Holbrook, they conducted social science surveys of Chicago's 19th ward. In 1895, they mapped this data to support improvements in sanitation and living conditions for the ethnic enclaves they served.
Satire. "Perception-based cartography" uses people's knowledge of accurate geographical data to show distortions that make arguments about people's preferences and blindspots. For instance, a 1908 “Map of the United States as seen by the Finance Committee of the United States Senate,” calls out politicians for focusing their spending on eastern cities at the expense of the rest of the nation.
Creation. Usually, a visualization represents information that exists in a different form. But in the case of a man-made island built for the 1939 Golden Gage International Exposition, the physical landscape was designed to be its own map: the information and the visualization were one and the same.
Contextualization. When multiple visualizations of different types of data appear together, users -- from airplane pilots reading gauges to city officials looking at statistics and demography -- get a complex picture from which to make operational decisions. The word "dashboard" has a long history, from its 1846 original definition as a mud guard at the front of a sleigh to its 1990 meaning as a "screen giving a graphical summary of various types of information."
Transparency. By 2021, many people expected to get the most up-to-date information about COVID-19 through data visualizations. The graphical displays were often frustratingly inadequate for guiding individual health decision-making, but nonetheless important for keeping citizens informed about the pandemic on the national and global scale.