If a bank is creating new paper notes, what sizes should it issue to make transactions easiest for the public? This is one example of an type of problem I've become interested in. It is basically about how to divide a scale into discrete intervals, under various constraints. It seems very abstract, but it actually has a lot to do with real-world design and engineering.
Graphs and charts often mislead by obscuring the unreliability of their source data. But even if a graph-maker wants to do better, it can be hard to present such information intelligibly, without long or technical sidebars. Here is one approach for visually displaying both the primary data, and their reliability, in one graph.