A statistical map with shading

Reproduced from Cleveland, W.S. & Mcgill, R. 1984. Graphical Perception - Theory, Experimentation, and Application to the Development of Graphical Methods. Journal of the American Statistical Association, 79, (387) 531-554.

The statistical map shows the useof grey shading to show the difference in values, here the murder rates per 100,000 people in the population in the United States of America in year 1978.Conventionally as it was done here, darker shades indicate higher values (rates). Users of the statistical map would need to recognise what the different shapes represent. Similarly, the grey shading can be replaced with different colours or varying colour saturations.