This article details the results of a written test designed to reveal how education majors construct and evaluate histograms and frequency polygons. Included is a description of the mistakes made by the students which shows how they tend to confuse histograms with bar diagrams, incorrectly assign data along the Cartesian axes and experience difficulties in constructing the frequency polygon.
A. Bruno and M.C. Espinel have written an article called Construction and evaluation of histograms in teacher training. The article was published in International Journal of Mathematical Education in Science and Technology a couple of days ago. Their study shows, among other things, that students confuse histograms with bar diagrams. Here is their abstract: