Faceting a plot into a Trellis Plot.
A Trellis plot (or small multiple) is a series of similar plots that displays different subsets of the same data, facilitating comparison across subsets. Adding a field to the
row channel produces a trellis plot that facets the plots into columns or rows respectively.
Below are three histograms for the horsepower of cars. Each chart shows the histogram for one origin (Europe, Japan, and USA).
You can find more examples in the example gallery.
column represent actual data fields that are used to partition the data, they cannot encode constant
value. Also, you should not facet by quantitative fields unless they are binned, or temporal fields unless you use