marks Property
Marks visually encode data using geometric primitives.
General JSON format:
A Vega marks
specification includes the following properties:
Each marks property is associated with the specified data property.
Marks are rendered in marks property array order.
Marks property values can be constants or as data references. You can use the scales property to transform marks property values to the visualization area.
Apply the x
and y
scales to the x
and y
database table columns to scale the data to the visualization area width and height. For example:
marks Properties
type
Marks must include a type
property that specifies the geometric primitive to use to render the data.
points Type
Specify x
and y
coordinate values using either constants, or domain and range values of a data
reference. If the from
property is not specified, the x
and y
properties
fields must be constants.
points Examples
Define a point with size, color, and opacity:
Associate the points
geometric primitive with tweets
data items.
lines Type
Specifying the data
format
property as lines
causes the rendering engine to assume a lines
database table layout and to extract line-related columns from the table.
Specify x
and y
coordinate values using either constants, or domain and range values of a data
reference. If the from
property is not specified, the x
and y
properties
fields must be constants.
lines Example
polys Type
The polys
type renders data as a polygon.
When the data
format property is polys
, the rendering engine assumes a polys
database table layout and extracts the poly-related columns from the table. A polys
database table layout implies that the first data column is the vertex x- and y-positions. The vertices are interleaved x and y values, such that vertex[0] = vert0.x
, vertex[1] = vert0.y
, vertex[2] = vert1.x
, and vertex[3] = vert1.y
, for example. The next three positions of a polys
database table are the triangulated indices, and line loop and drawing information for unpacking multiple, associated polygons that can be packed as a single data item.
polys Example
symbol Type
The symbol
marks type renders data as one of the supported shapes.
Currently, in symbol
mark types, strokes are not visible beneath other marks, regardless of opacity settings.
Specify x
and y
coordinate values using either constants or domain and range values of a data
reference. If the from
property is not specified, the x
and y
properties
fields must be specified using constant values.
symbol
Examples
symbol
ExamplesThe following example defines symbol mark types including fill, stroke, and general opacity properties:
from
The from
field specifies the input database table to use.
Example
Use the tweets
database table for marks input data.
If from
is not specified, the data source is implicitly a single point with the value defined in the points
properties.
properties
The properties
property specifies type-dependent visual encoding that define the position and appearance of mark instances. The property value is specified using one of the Value Reference options.
Typically, a single mark instance is generated per input data element, except for polys
, which uses multiple data elements to represent a line or area shape.
The following table describes the various marks properties
and lists the types for which the property is valid.
Value Reference
A value reference describes how to specify marks properties
values. The value can be a constant or data object reference:
Examples:
Statically set the point fillColor
and size
.
For the x
marks property, apply the x
scale transform to the implicit x-coordinate data column.
Field Reference
A field reference is either a string literal or an object. For object values, the following properties are supported:
Color Value Reference
Typically, color values are specified as a single RGB color value. To specify specific color fields or use a different color space, use one of the following color value reference formats:
Examples
Set the red and blue channels of an RGB color as constants, and uses a scale transform to determine the green channel:
Use the rgb
color space for the color
field:
transform
The transform
object specifies any Vega projections to be applied to the mark. Each transform is specified as a key:value pair in the transform
object:
The value references an existing Vega object by name.
For example, the following transform references the projection my_mercator_projection
defined in the top-level Vega projections
property.
Currently, the only supported transform is projection
.
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