manual: add the 'Typographic Units' page

Typographic Units

=================

A number of options take a typographic unit as an argument. These options are

marked with the type `<measure>`.

The following typographic units are currently accepted:

  - `px` -- Pixels

  - `pt` -- Typographic points

  - `em` -- Typographic "em" size

  - `%`  -- Relative to the enclosing element

  - User units on an arbitrary scale

#### Examples

    ;; 10 pixels

    10px

    ;; 16 points

    16pt

    ;; 2 "em"

    2em

    ;; 50% of the parent

    50%

"Responsive" Charts

-------------------

Users are encouraged to specify typographic measurements using the "em" or "pt"

units. Doing so will allow you to scale the output resolution of the chart using

the global `dpi` option.

Mapping input data to typographic units

---------------------------------------

Sometimes you may want to map input data to typographic units. Once instance where

this is useful is when creating a scatterplot where the point size depends on

some input datum. Mapping from input data to typographic units is accomplished

using the `<measure-map>` construct. The following mappings are currently

supported:

    ;; map a value in the range [0..1] linearly to min..max

    (linear <min> <max>)

#### Examples

    ;; map 0..1 to 12pt..20pt

    (linear 12pt 20pt)

          title: "Color Maps"

          url: "/documentation/color-maps"

          file: "reference/color_maps"

        -

          title: "Typographic Units"

          url: "/documentation/typographic"

          file: "reference/typographic"

        -

          title: "Scale Types"

          url: "/documentation/scale-types"