Line Mark
Line marks are stroked paths with constant width, defined by an ordered set of (x, y) coordinates. While line marks default to using straight line segments, different interpolation methods can be used to create smoothed or stepped paths. Line marks are commonly used to depict trajectories or change over time.
Example
Type-Specific Mark Properties
| Property | Type | Description |
|---|---|---|
| interpolate | String | The interpolation method to use. One of basis, bundle, cardinal, catmull-rom, linear, monotone, natural, step, step-after, step-before. The default is linear. You can find explanations for these line interpolators in the d3-shape documentation. |
| tension | Number | The tension value in the range [0, 1] to parameterize bundle (default 0.8), cardinal (default 0) or catmull-rom (default 0.5) interpolation. |
| defined | Boolean | A boolean flag indicating if the current data point is defined. If false, the corresponding line segment will be omitted, creating a “break”. |
Note: If a data point on a line is surrounded by points with defined: false, it may not be visible. Use a strokeCap of round or square to ensure a visible point is drawn.
General Mark Properties
| Property | Type | Description |
|---|---|---|
| x | Number | The primary x-coordinate in pixels. |
| x2 | Number | The secondary x-coordinate in pixels. |
| xc | Number | The center x-coordinate. Incompatible with x and x2. |
| width | Number | The width of the mark in pixels, if supported. |
| y | Number | The primary y-coordinate in pixels. |
| y2 | Number | The secondary y-coordinate in pixels. |
| yc | Number | The center y-coordinate. Incompatible with y and y2. |
| height | Number | The height of the mark in pixels, if supported. |
| opacity | Number | The mark opacity from 0 (transparent) to 1 (opaque). |
| fill | Color | The fill color. |
| fillOpacity | Number | The fill opacity from 0 (transparent) to 1 (opaque). |
| stroke | Color | The stroke color. |
| strokeOpacity | Number | The stroke opacity from 0 (transparent) to 1 (opaque). |
| strokeWidth | Number | The stroke width in pixels. |
| strokeCap | String | The stroke cap for line ending style. One of butt (default), round or square. |
| strokeDash | Number[ ] | An array of [stroke, space] lengths for creating dashed or dotted lines. |
| strokeDashOffset | Number | The pixel offset at which to start the stroke dash array. |
| strokeJoin | String | The stroke line join method. One of miter (default), round or bevel. |
| strokeMiterLimit | Number | The miter limit at which to bevel a line join. |
| blend | String | The color blend mode for drawing an item on its current background. Any valid CSS mix-blend-mode value except for "normal" can be used. The default value null maps to blend values of normal for SVG and source-over for Canvas. |
| cursor | String | The mouse cursor used over the mark. Any valid CSS cursor type can be used. |
| href | URL | A URL to load upon mouse click. If defined, the mark acts as a hyperlink. |
| tooltip | Any | The tooltip text to show upon mouse hover. If the value is an object (other than a Date or an array), then all key-value pairs in the object will be shown in the tooltip, one per line (e.g., "key1: value1\nkey2: value2"). Array values will be shown in brackets [value1, value2, ...]. Other values will be coerced to strings. Nested object values will not be recursively printed. |
| zindex | Number | An integer z-index indicating the layering order of sibling mark items. The default value is 0. Higher values (1) will cause marks to be drawn on top of those with lower z-index values. Setting the z-index as an encoding property only affects ordering among sibling mark items; it will not change the layering relative to other mark definitions. Unlike the mark-level sort property, zindex changes the rendering order only; it does not otherwise change mark item order (such as line or area point order). The most common use of zindex is to ensure that a mark is drawn over its siblings when selected, such as by mouse hover. |
For marks that support width and height settings (including rect and area), the horizontal dimensions are determined (in order of precedence) by the x and x2 properties, the x and width properties, the x2 and width properties, or the xc and width properties. If all three of x, x2 and width are specified, the width value is ignored. The y, y2, yc and height properties are treated similarly.
For marks that do not support width or height (including path and arc) similar calculations are applied, but are only used to determine the mark’s ultimate x and y position. The width property may affect the final x position, but otherwise is not visualized.
When using multiple spatial properties along the same dimension (x and x2, or y and y2), the properties must be specified in the same encoding set; for example, all within "enter": {...} or all within "update": {...}. Dividing the properties across encoding sets can cause unexpected behavior.
Accessibility Properties ≥ 5.11
Accessibility properties are used to determine ARIA (Accessible Rich Internet Applications) attributes when using Vega to render SVG output. Vega will automatically generate ARIA attributes for container SVG <g> elements for each mark definition; this can be configured using mark-level accessibility properties. Use the properties below in encode sets to add ARIA attributes for individual mark items.
| Property | Type | Description |
|---|---|---|
| aria | Boolean | A boolean flag (default true) indicating if ARIA attributes should be included (SVG output only). If false, the “aria-hidden” attribute will be set on the output SVG element, removing the mark item from the ARIA accessibility tree. |
| description | String | A text description of the mark item for ARIA accessibility (SVG output only). If specified, this property determines the “aria-label” attribute. |