Improving continuously requires to be truly aware of one's current condition. A lot comes "for free" when using Kanban (visualization policies, feedback policies and the measure of flow). The Kanban board gives a "snapshot" of the current situation, but a time aspect is needed to better understand trends and evolution. That is where the charts and graphs come in. Current Kanban best practices recommend the usage of certain graphs like the Cumulative Flow Diagram (CFD) and Statistical Control Chart.
Here I share a few simple examples of the graphs produced by some of the team I have the chance to coach and what the graph can tells us on the team (this content is an extract from the "Kanban Kick-start Field Guide" to be published soon). For help on how to produce these charts, check the invaluable Excel how-tos by Håkan Forss on CFD, Control Chart and Spectral Analysis.