The biomedical literature is a not only a tool to find the newest answers, but is also a record of what researchers and clinicians have concerned themselves with over the decades. PubMed does a thorough job of covering this vast literature, as it contains citations from around 26,000 journals published around the world. MEDLINE and OLDMEDLINE take us all the way back to 1946, though there are also a smattering of PubMed Central citations between the nineteenth century and then.
PubMed records are indexed to different degrees of granualarity, but every citation has a publication date. Results for each year are compared with that year's total in the database, and the resulting proportion is plotted on a graph. We use proportions instead of raw numbers, as the the astonishing increase of the biomedical literature over time makes a bare count less illustrative of changes for any given search.
When you enter a search into the search bar, you are searching PubMed directly, meaning that your search is parsed by NCBI's automatic term mapping algorithm. If something doesn't make sense, be sure to launch the search in PubMed (by clicking on any year on the graph) and examine the "Search details" box.
Use caution when interpreting the results of a Medical Subject Heading search, as older records are not typically re-indexed after a new heading is added to the thesaurus.
As PubMed by Year uses the "Results by Year" CSV file generated by PubMed, it does not work for searches that return fewer than 500 total results (because that file doesn't get generated for smaller result sets).
PubMed by Year is lashed together with bailing wire and jQuery. NCBI does not have an easy way to grab by-year counts for a search (short of launching 50+ calls to the API), so a custom webservice is used to fetch them. Google Charts is used to draw the charts and responsive layout is made easier with Bootstrap.
You can find the source code for this application at GitHub.
Feel free to use this tool as you wish, but if you use PubMed by Year for publication, I'd appreciate a citation:
Sperr E. PubMed by Year [Internet]. 2016 [cited your_date_here]. Available from http://esperr.github.io/pubmed-by-year/
Want to have even more fun with tools like this? Check out Visualizing PubMed.