I think a question on a lot of people's lips is "if we put the effort in to retrain and use this guide, how much can I expect to earn?" All well and good saying this is what employers want in an interview, but we have to talk remuneration too :)
In the bottom left of the compensation graph, there is a non-zero percentage of people earning less than 0 USD per year. I guess they used a smoothing function fit to the data points. The alternative is that some people are paying their employers some dollars for the privilege of working for them :p