This function is modeled on the element_* functions used in
ggplot2 to specify graphical properties in themes. It is primarily
used to create the value on the right-hand side of an assignment involving
the props<- group of setter functions.
A horizontal or vertical rule (hvrule) is actually drawn as long, narrow
rectangle, with a line centered inside it. The narrowness of the
rectangle, and thus how much space the hvrule adds to the table, is
controlled by space. The thickness of the line inside the
rectangle is controlled by size. Setting linetype to 0
means no line will be drawn, but the enclosing rectangle will be. In that
way hvrules can be used to insert extra blank space between rows or
columns of a table.
Quantitative properties size and space may be specified
using the ggplot2 function rel(). This function indicates
that the value is to be interpreted as a multiplier to be applied to
whatever the current value of the property is. For example
space=rel(1.2) specifies that the space property of an entry
is to be increased by 20% from its current value.