Understanding the frequency of occurrences over time of an event or value is an important aspect of scientific research. The histogram is a graphical analysis that allows the cartographer to easily display and see how trends happen with relation to time and a variable. There is an x-axis that is considered some unit of time and there is a y-axis that is the variable in study. A color coded system can also be used to distinguish another variable considered in the study. The map displayed here is a histogram relating the years of 1891 to 1970 through 10 year ranges to US immigration rates from Northern Europe. The colors represent the countries that each population came from. It appears that the Swedish immigrants entered into the United States mostly between 1891 and 1910, however dropped off soon after as many other immigration nationalities did according to this data.
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