Layers

A gnuplot plot is built up by drawing its various components in a fixed order. This order can be modified by assigning some components to a specific layer using the keywords behind, back, or front. For example, to replace the background color of the plot area you could define a colored rectangle with the attribute behind.

set object 1 rectangle from graph 0,0 to graph 1,1 fc rgb "gray" behind

The order of drawing is

behind
back
the plot itself
the plot legend (`key`)
front

Within each layer elements are drawn in the order

objects (rectangles, circles, ellipses, polygons) in numerical order
labels in numerical order
arrows in numerical order

In the case of multiple plots on a single page (multiplot mode) this order applies separately to each component plot, not to the multiplot as a whole.