When I worked on the MATLAB Plot Catalog (see description here), I needed a thumbnail for each plot type. Users might know what kind of plot they needed, but they wouldn't necessarily know what the MATLAB engineers had named it. The thumbnails also served as a "visual tour" of MATLAB's plot capabilities, which had gone unappreciated for years by most users!
The icons needed to capture the uniqueness of each plot type, but of course, they also had to hang together as a coherent set. These are listed according to the groupings I devised for the plot types: