ggplot2: ribbon changes fill when ymin is greater than ymax
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I have two time series data, and one goes higher than other for a certain period and then reverses the order in another period.  Can I use geom_ribbon to fill the gap between two lines and then use different color (e.g. gray if `ts2 > ts1` and pink if `ts2 < ts1`)?

it appears to me that ggplot seems doing implicitly like `aes(ymin=pmin(ymin,ymax), ymax=pmax(ymin,ymax))`, doesn't care which of two are actually greater.
