add documentation on colors and color maps

Color Maps

==========

The `<color-map>` setting allows you to map input data to colors. These are the

currently implemented mapping methods:

  - `discrete` -- Read integer values from the input file and map them to a discrete color palette

  - `linear` -- Read numeric values from the input file and linearly map them to a color gradient

  - `raw` -- Parse colors in hex format from the input file

#### Syntax:

    ;; a discrete color map

    (discrete (<color>...) <data-ref>)

    ;; a discrete color map using a default palette

    (discrete <color-palette-name> <data-ref>)

    ;; a linear color map

    (linear (<color> <color>) <data-ref>)

    ;; a linear "default" color map

    (linear <color-map-name> <data-ref>)

    ;; a "raw" color map

    (raw <data-ref>)

#### Examples:

    ;; map the values of 'mycolorcolumn' to {red, green, blue}.

    ;; values in 'mycolorcolumn' should be in the set {1, 2, 3}

    (discrete (#f00 #0f0 #00f) (csv "myfile.csv" "mycolorcolumn"))

    ;; map the values of 'mycolorcolumn' to [red..blue].

    ;; values in 'mycolorcolumn' should be in [0..1]

    (linear #f00 #00f (csv "myfile.csv" "mycolorcolumn"))

    ;; read hex color codes from 'mycolorcolumn'

    ;; values in 'mycolorcolumn' should be hex color codes (e.g. "#f00")

    (raw (csv "myfile.csv" "mycolorcolumn"))

List of built-in color maps

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

Colors

======

The `<color>` setting allows you to configure colors. The following ways to

specify colors are supported:

    ;; specify a color by hex code or reference a color from the color palette

    <color-code>

    ;; specify a color by rgb. components should be in the range 0..1

    (rgb <r> <g> <b>)

    ;; specify a color by rgba. components should be in the range 0..1

    (rgba <r> <g> <b> <a>)

#### Examples:

    ;; set the color to green

    color #f00

    ;; set the color to green

    color #ff0000

    ;; set the color to green, 50% alpha

    color #ff000080

    ;; set the color to red

    color (rgb 1.0 0.0 0.0)

    ;; set the color to red, 50% alpha

    color (rgba 1.0 0.0 0.0 0.5)

    ;; usage color number '3' from the color palette

    color 3

Color Palettes

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

The color palette allows you to easily reference colors that are frequently used

in the chart. Colors from the color palette are referenced by one-base index

(see above).

You can use one of the built-in color palettes (see below) or define your own

color palette using the `set` directive:

    (set color-palette (<color>...))

For example:

    (set color-palette (#06c #c06 #60c))

List of default color palettes

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

          title: "Example Gallery"

          url: "/examples"

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        -

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          title: "Fonts"

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          file: "reference/fonts"

  #    #-

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  #    #-

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  #    #  url: "/documentation/line-styles"

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  #    #-

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  #    #-

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      #-

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      #  url: "/documentation/line-styles"

      #  file: "line_styles"

  -

    title: "Elements"

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