Marks & Geometries

While Encodings and Scales define the mathematical relationship between data and space, the Mark is the physical manifestation of that relationship. A Mark is the geometric primitive used to represent a data point or a set of data points.

The Role of a Mark

In Charton, a Mark is not just a drawing instruction; it is a Template that knows how to interpret resolved aesthetic values (pixels, hex codes, shapes) into final geometry.

As defined in the Mark trait, every mark type in Charton:

  1. Identifies itself: Each mark has a unique string identifier (e.g., "point", "bar").
  2. Provides Defaults: Marks define fallback values for properties like stroke, opacity, and shape if they are not explicitly mapped to data.
  3. Determines Rendering Logic: Different marks require different drawing strategies—a Point is a single coordinate, while an Area is a complex polygon.

Common Mark Types

Charton provides a rich set of marks to cover various visualization needs:

Point Mark (mark_point)

The simplest mark, representing each data row as an individual geometric shape.

  • Dimensions: Primarily uses X and Y.
  • Aesthetics: Heavily utilizes Shape, Size, and Color.
  • Use Case: Scatter plots and bubble charts.

Line Mark (mark_line)

Connects data points in a specific order (usually by the X-axis) to show trends.

  • Connectivity: Unlike points, the Line mark treats a sequence of rows as a single continuous path.
  • Visuals: Focuses on stroke_width and color.

Bar Mark (mark_bar)

Represents data as rectangles extending from a baseline.

  • Physicality: Bars have "width." Charton calculates this width based on the CoordLayout (Chapter 1.4) to ensure bars don't overlap unless intended.
  • Intervals: Uses X, Y (height), and sometimes Y2 (for ranged bars).

Area Mark (mark_area)

Similar to a line but filled between a baseline (Y2) and the data value (Y).

  • Topology: Highlighting the volume between two series or between a series and the zero-axis.

Specialized Marks

  • Rule & Tick: Used for annotations or error margins.
  • Rect: Drawing arbitrary rectangles based on coordinate pairs.
  • Text: Placing strings directly into the coordinate space.

From Mark to Geometry: The Renderer

Behind every Mark lies a corresponding Renderer. When Charton enters the "Realization" phase (Chapter 3.4), it translates the mark's configuration into physical geometry:

  • PointElement: A simple struct containing x, y, shape, size.
  • PathConfig: A collection of points and stroke properties used for Lines and Areas.
  • RectConfig: Defined by x, y, width, height for Bars and Histograms.

Marks and Categorical Stacking

One of Charton's advanced features is how Marks handle Stacking and Grouping.

As seen in the MarkBar implementation, when multiple series exist on the same X-coordinate:

  • Stacked: The Y value of the second mark starts at the Y end-point of the first.
  • Grouped (Side-by-Side): The X position is offset by a fraction of the "Slot width," ensuring bars are placed next to each other without manual coordinate calculation.

Visual Consistency (The Mark Trait)

In mark.rs, the Mark trait ensures that all geometric primitives share a common interface. This allows the LayeredChart to treat a PointChart and a LineChart identically during the high-level orchestration phase, even though their low-level draw calls are completely different.

Key Takeaways

  • Marks are the "ink" on the page.
  • Mark choice changes the narrative of the data (e.g., a Line implies a trend, while a Bar implies a comparison).
  • Geometric resolution is the final step where abstract scales are converted into physical shapes (Circles, Rects, Paths).