Marker definition and scope
A marker defines a graphic drawn on vertices of a line or link mark.
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markers
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Marker options: markerStart, markerMid, markerEnd, marker
The supported marker options are: markerStart (marker for starting point of a line segment), markerMid (marker for any intermediate point), markerEnd (marker for end point), and marker (shorthand for setting marker on all points).
Named markers available
The named markers supported are: none (default, no marker), arrow (arrowhead with auto orientation), arrow-reverse (arrowhead with auto-start-reverse orientation), dot (filled circle without stroke, 2.5px radius), circle or circle-fill (filled circle with white stroke, 3px radius), circle-stroke (hollow circle with colored stroke and white fill, 3px radius), tick (small opposing line, added in 0.6.12), tick-x (small horizontal line, added in 0.6.12), and tick-y (small vertical line, added in 0.6.12).
Marker parameter defaults and function behavior
If marker is true, it defaults to circle. If marker is a function, it will be called with a given color and must return an SVG marker element.
Marker color inheritance and arrow orientation
The primary color of a marker is inherited from the stroke of the associated mark. The arrow marker is automatically oriented such that it points in the tangential direction of the path at the position where the marker is placed. Circle markers are centered around the given vertex.
Marker placement with non-linear curves
For lines whose curve is not linear, markers are not necessarily drawn at the data positions given by x and y; marker placement is determined by the (possibly Bézier) path segments generated by the curve. To ensure that symbols are drawn at a given x and y position, consider using a dot mark instead.