There are really two questions. The first is: how does one get BarChart to generate x coordinates. That is, ListPlot takes a list of pairs, but BarChart takes a list of y-values only. Is there any way to impart x values to it?

Second question (which is related, because I am using BarChart so I can use ColorFunction) is: how do I invert ColorFunction. That is, "DarkRainbow" makes high values red, low values green. I want it exactly the other way around. What's the trick?