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Variables that can be placed in layergroup's definition (SQL or CartoCSS of any layer). Placeholders need to be defined with a type and a default value for MapConfigs. See details about defining a MapConfig type for Layergroup configurations. Valid placeholder names start with a letter and can only contain letters, numbers, or underscores. They have to be written between the <%= and %> strings in order to be replaced inside the Named Maps API. Example:<%= my_color %> The set of supported placeholders for a template need to be explicitly defined with a specific type, and default value, for each placeholder. Placeholder default values will be used whenever new values are not provided as options, at the time of creation on the client. They can also be used to test the template by creating a default version with new options provided. When using templates, be very careful about your selections as they can give broad access to your data if they are defined loosely.

Extra keys to specify the view area for the map. It can be used to have a static preview of a Named Map without having to instantiate it. It is possible to specify it with center + zoom or with a bounding box bbox. Center+zoom takes precedence over bounding box. Also it is possible to choose which layers are visible or not with preview_layers indicating its visibility by layer index or id (visible by default).