Sometimes, people want to plot things in three dimensions. Others have spoken more eloquently than I could on the potential problems with plotting multiple two-dimensional relationships in a two-dimensional medium with an artificial three-dimensional visual effect. Fortunately, a third dimension can be easily represented without a faux-third dimension, with color, shape, or a small multiples approach.
Just as fortunately, one of the main advantages of the ggplot2 plotting framework is the ability to easily create small multiple plots across separate facets. The Gist below illustrates three easy alternatives to a “3-D” plot for a time series: colored lines, faceted series, and, most usefully, faceted series with reference lines.
The latter approach is probably the best, in that it doesn’t rely on color reproduction or discrimination, and it allows for clear comparison across facets. It is not always obvious how to make “background” geometries across all facets, but it is usually simple, as it is in the example below.