This is the best population visualization I've ever seen. It's striking that spikes turned out to be so superior to heatmaps. It's possible to intuit the whole dataset at a glance, whereas heatmaps force you to constantly worry about the meaning of every color in relation to every other color.

It's obvious in retrospect why spikes are superior: precision. Spikes represent the underlying data much more accurately than colors, especially between datapoints.