Choropleth mapping is a common method of displaying trends in data across a chosen geographic area. These maps are commonly used in journalism to map election or poll results but can be useful for many fields from the humanities to natural sciences. For a more detailed explanation of choropleth mapping see the Penn State University guide here.
The MSU Libraries workshop will walk through how to produce a simple vector map using both choropleth and variable sizing to display two possibly related trends across the state of Michigan.