Diverging Bar Charts for Connected Collections

As springtime sprouts across Colorado, it’s a fine time to unearth some data on another statewide system that benefits all library users. The interlibrary loan system (ILL) connects Colorado libraries and their collections to ensure access to the largest possible pool of library materials. In fact, the existence of an ILL system is actually written into Colorado Library Law to guarantee resource sharing across Colorado. The Public Library Annual Report (PLAR) compiles two pieces of ILL data by asking Colorado public libraries how many items they borrow from and how many items they loan to other libraries through ILL. The reports vary from libraries who do not participate in any ILLs to libraries such as Boulder Public Library who reported loaning out 74,703 items and Longmont Public Library who reported borrowing 150,208 items through ILL in 2022.

The first diverging bar chart in this post groups libraries by legal service area (LSA) population to compare the number of ILLs loaned and borrowed. As you may expect, larger libraries generally have more capacity to loan ILLs and receive more requests for ILLs. However, the number of ILLs a library loans and borrows could also be affected by a multitude of other factors including the demographics or reading preferences of the communities the library serves and whether the library advertises the availability of ILLs. To adjust for LSA population differences, and explore additional factors that may impact a library’s participation in the ILL system, we’ll also take a look at ILLs by circulation per capita, materials expenditures per capita, and a handful of individual library comparisons. Before we dig in though, let’s first introduce diverging bar charts to ensure we’re interpreting the data correctly…

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