ggplot2时间序列的着色信封

I am plotting the results of 50 - 100 experiments. Each experiment results in a time series. I can plot a spaghetti plot of all time series, but what I'd like to have is sort of a density map for the time series plume. (something similar to the gray shading in the lower panel in this figure: I am plotting the results of 50 - 100 experimen