Marks

We saw in Encodings that the encode() method is used to map columns to visual attributes of the plot. The mark property is what specifies how exactly those attributes should be represented on the plot.

Altair provides a number of mark properties:

Mark Name	Method	Description	Example
area	mark_area()	A filled area plot.	Simple Stacked Area Chart
bar	mark_bar()	A bar plot.	Simple Bar Chart
circle	mark_circle()	A scatter plot with filled circles.	One Dot Per Zipcode
geoshape	mark_geoshape()	A geographic shape	Choropleth Map
line	mark_line()	A line plot.	Simple Line Chart
point	mark_point()	A scatter plot with configurable point shapes.	Faceted Scatter Plot with Linked Brushing
rect	mark_rect()	A filled rectangle, used for heatmaps	Simple Heatmap
rule	mark_rule()	A vertical or horizontal line spanning the axis.	Candlestick Chart
square	mark_square()	A scatter plot with filled squares.	N/A
text	mark_text()	A scatter plot with points represented by text.	Simple Bar Chart with Labels
tick	mark_tick()	A vertical or horizontal tick mark.	Strip Plot

In Altair, marks can be most conveniently specified by the mark_* methods of the Chart object, which take optional keyword arguments that are passed to MarkDef to configure the look of the marks.

For example, the following uses mark_circle() with additional arguments to represent points as red semi-transparent filled circles:

import altair as alt
from vega_datasets import data

url = data.cars.url

alt.Chart(url).mark_circle(
    color='red',
    opacity=0.3
).encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q'
)

The additional arguments to mark_*() methods are passed along to an associated MarkDef instance, which supports the following attributes:

Property	Type	Description
align	HorizontalAlign	The horizontal alignment of the text. One of "left", "right", "center".
angle	number	The rotation angle of the text, in degrees.
baseline	VerticalAlign	The vertical alignment of the text. One of "top", "middle", "bottom". Default value: "middle"
binSpacing	number	Offset between bars for binned field. Ideal value for this is either 0 (Preferred by statisticians) or 1 (Vega-Lite Default, D3 example style). Default value: 1
clip	boolean	Whether a mark be clipped to the enclosing group’s width and height.
color	string	Default color. Note that fill and stroke have higher precedence than color and will override color. Default value: ■ "#4682b4" Note: This property cannot be used in a style config.
cornerRadius	number	The radius in pixels of rounded rectangle corners. Default value: 0
cursor	Cursor	The mouse cursor used over the mark. Any valid CSS cursor type can be used.
dir	Dir	The direction of the text. One of "ltr" (left-to-right) or "rtl" (right-to-left). This property determines on which side is truncated in response to the limit parameter. Default value: "ltr"
dx	number	The horizontal offset, in pixels, between the text label and its anchor point. The offset is applied after rotation by the angle property.
dy	number	The vertical offset, in pixels, between the text label and its anchor point. The offset is applied after rotation by the angle property.
ellipsis	string	The ellipsis string for text truncated in response to the limit parameter. Default value: "…"
fill	string	Default Fill Color. This has higher precedence than config.color Default value: (None)
fillOpacity	number	The fill opacity (value between [0,1]). Default value: 1
filled	boolean	Whether the mark’s color should be used as fill color instead of stroke color. Default value: true for all marks except point and false for point. Applicable for: bar, point, circle, square, and area marks. Note: This property cannot be used in a style config.
font	string	The typeface to set the text in (e.g., "Helvetica Neue").
fontSize	number	The font size, in pixels.
fontStyle	FontStyle	The font style (e.g., "italic").
fontWeight	FontWeight	The font weight. This can be either a string (e.g "bold", "normal") or a number (100, 200, 300, …, 900 where "normal" = 400 and "bold" = 700).
href	string	A URL to load upon mouse click. If defined, the mark acts as a hyperlink.
interpolate	Interpolate	The line interpolation method to use for line and area marks. One of the following: "linear": piecewise linear segments, as in a polyline. "linear-closed": close the linear segments to form a polygon. "step": alternate between horizontal and vertical segments, as in a step function. "step-before": alternate between vertical and horizontal segments, as in a step function. "step-after": alternate between horizontal and vertical segments, as in a step function. "basis": a B-spline, with control point duplication on the ends. "basis-open": an open B-spline; may not intersect the start or end. "basis-closed": a closed B-spline, as in a loop. "cardinal": a Cardinal spline, with control point duplication on the ends. "cardinal-open": an open Cardinal spline; may not intersect the start or end, but will intersect other control points. "cardinal-closed": a closed Cardinal spline, as in a loop. "bundle": equivalent to basis, except the tension parameter is used to straighten the spline. "monotone": cubic interpolation that preserves monotonicity in y.
limit	number	The maximum length of the text mark in pixels. The text value will be automatically truncated if the rendered size exceeds the limit. Default value: 0, indicating no limit
line	anyOf(boolean, OverlayMarkDef)	A flag for overlaying line on top of area marks, or an object defining the properties of the overlayed lines. If this value is an empty object ({}) or true, lines with default properties will be used. If this value is false, no lines would be automatically added to area marks. Default value: false.
opacity	number	The overall opacity (value between [0,1]). Default value: 0.7 for non-aggregate plots with point, tick, circle, or square marks or layered bar charts and 1 otherwise.
orient	Orient	The orientation of a non-stacked bar, tick, area, and line charts. The value is either horizontal (default) or vertical. For bar, rule and tick, this determines whether the size of the bar and tick should be applied to x or y dimension. For area, this property determines the orient property of the Vega output. For line and trail marks, this property determines the sort order of the points in the line if config.sortLineBy is not specified. For stacked charts, this is always determined by the orientation of the stack; therefore explicitly specified value will be ignored.
point	anyOf(boolean, OverlayMarkDef, [‘transparent’])	A flag for overlaying points on top of line or area marks, or an object defining the properties of the overlayed points. If this property is "transparent", transparent points will be used (for enhancing tooltips and selections). If this property is an empty object ({}) or true, filled points with default properties will be used. If this property is false, no points would be automatically added to line or area marks. Default value: false.
radius	number	Polar coordinate radial offset, in pixels, of the text label from the origin determined by the x and y properties.
shape	string	The default symbol shape to use. One of: "circle" (default), "square", "cross", "diamond", "triangle-up", or "triangle-down", or a custom SVG path. Default value: "circle"
size	number	The pixel area each the point/circle/square. For example: in the case of circles, the radius is determined in part by the square root of the size value. Default value: 30
stroke	string	Default Stroke Color. This has higher precedence than config.color Default value: (None)
strokeCap	StrokeCap	The stroke cap for line ending style. One of "butt", "round", or "square". Default value: "square"
strokeDash	array(number)	An array of alternating stroke, space lengths for creating dashed or dotted lines.
strokeDashOffset	number	The offset (in pixels) into which to begin drawing with the stroke dash array.
strokeJoin	StrokeJoin	The stroke line join method. One of "miter", "round" or "bevel". Default value: "miter"
strokeMiterLimit	number	The miter limit at which to bevel a line join.
strokeOpacity	number	The stroke opacity (value between [0,1]). Default value: 1
strokeWidth	number	The stroke width, in pixels.
style	anyOf(string, array(string))	A string or array of strings indicating the name of custom styles to apply to the mark. A style is a named collection of mark property defaults defined within the style configuration. If style is an array, later styles will override earlier styles. Any mark properties explicitly defined within the encoding will override a style default. Default value: The mark’s name. For example, a bar mark will have style "bar" by default. Note: Any specified style will augment the default style. For example, a bar mark with "style": "foo" will receive from config.style.bar and config.style.foo (the specified style "foo" has higher precedence).
tension	number	Depending on the interpolation type, sets the tension parameter (for line and area marks).
text	string	Placeholder text if the text channel is not specified
theta	number	Polar coordinate angle, in radians, of the text label from the origin determined by the x and y properties. Values for theta follow the same convention of arc mark startAngle and endAngle properties: angles are measured in radians, with 0 indicating “north”.
thickness	number	Thickness of the tick mark. Default value: 1
tooltip	–	The tooltip text to show upon mouse hover.
type	Mark	The mark type. One of "bar", "circle", "square", "tick", "line", "area", "point", "geoshape", "rule", and "text".
x2Offset	number	Offset for x2-position.
xOffset	number	Offset for x-position.
y2Offset	number	Offset for y2-position.
yOffset	number	Offset for y-position.

Marks can also be configured globally using chart-level configurations; see Mark and Mark Style Configuration for details.