With the threshold plugin, you can apply a specific color to the part of a data series below a threshold. This is can be useful for highlighting negative values, e.g. when displaying net results or what's in stock.

For each point of a filled curve, you can specify an arbitrary bottom. As this example illustrates, this can be useful for plotting percentiles. If you have the data sets available without appropriate fill bottoms, you can use the fillbetween plugin to compute the data point bottoms automatically.