To display a space of specified width and/or height, use a display
specification of the form (space . props), where
props is a property list (a list of alternating properties and
values). You can put this property on one or more consecutive
characters; a space of the specified height and width is displayed in
place of all of those characters. These are the properties you
can use in props to specify the weight of the space:
:width width
If width is an integer or floating point number, it specifies
that the space width should be width times the normal character
width. width can also be a pixel width specification
(see Pixel Specification).
:relative-width factor
Specifies that the width of the stretch should be computed from the
first character in the group of consecutive characters that have the
same display property. The space width is the width of that
character, multiplied by factor.
:align-to hpos
Specifies that the space should be wide enough to reach hpos.
If hpos is a number, it is measured in units of the normal
character width. hpos can also be a pixel width
specification (see Pixel Specification).
You should use one and only one of the above properties. You can
also specify the height of the space, with these properties:
:height height
Specifies the height of the space.
If height is an integer or floating point number, it specifies
that the space height should be height times the normal character
height. The height may also be a pixel height specification
(see Pixel Specification).
:relative-height factor
Specifies the height of the space, multiplying the ordinary height
of the text having this display specification by factor.
:ascent ascent
If the value of ascent is a non-negative number no greater than
100, it specifies that ascent percent of the height of the space
should be considered as the ascent of the space—that is, the part
above the baseline. The ascent may also be specified in pixel units
with a pixel ascent specification (see Pixel Specification).
Don't use both :height and :relative-height together.
The :width and :align-to properties are supported on
non-graphic terminals, but the other space properties in this section
are not.