Segment

Layers are declared with the DRAW clause. Read the documentation for this clause for a thorough description of how to use it.

The segment layer is used to create line segments between two endpoints. If differs from lines and paths in that it connects just two points rather than many. Data is expected to be in a different shape, with 4 coordinates for the start (x, y) and end (xend, yend) points on a single row.

Aesthetics

The following aesthetics are recognised by the segment layer.

Required

• x: Position along the x-axis of the start point.
• y: Position along the y-axis of the end point.
• xend*: Position along the x-axis of the end point.
• yend*: Position along the y-axis of the end point.

* Only one of xend and yend is required. If one is missing, it takes on the value of the start point.

Optional

• colour/stroke: The colour of the line.
• opacity: The opacity of the line.
• linewidth: The width of the line.
• linetype: The type of the line, i.e. the dashing pattern.

Settings

The segment layer has no additional settings.

Data transformation

The segment layer does not transform its data but passes it through unchanged.

Data transformation

Examples

Segments are useful when you have known start and end points of the data. For example in a graph

WITH edges AS (
  SELECT * FROM (VALUES
      (0, 0, 1, 1, 'A'),
      (1, 1, 2, 1, 'A'),
      (2, 1, 3, 0, 'A'),
      (0, 3, 1, 2, 'B'),
      (1, 2, 2, 2, 'B'),
      (2, 2, 3, 3, 'B'),
      (1, 1, 1, 2, 'C'),
      (2, 2, 2, 1, 'C')
  ) AS t(x, y, xend, yend, type)
)
VISUALISE x, y, xend, yend FROM edges
  DRAW segment MAPPING type AS stroke

You can use segments as part of a lollipop chart by anchoring one of the ends to 0. Note that xend is missing and has taken up the value of x.

SELECT ROUND(bill_dep) AS bill_dep, COUNT(*) AS n 
  FROM ggsql:penguins 
  GROUP BY ROUND(bill_dep)

VISUALISE bill_dep AS x, n AS y
  DRAW segment MAPPING 0 AS yend
  DRAW point

By overlaying a thick line on a thin line, you can create a candlestick chart.

SELECT
  FIRST(Date) AS date,
  FIRST(temp) AS open,
  LAST(temp) AS close,
  MAX(temp) AS high,
  MIN(temp) AS low,
  CASE
    WHEN FIRST(temp) > LAST(temp) THEN 'colder'
    ELSE 'warmer'
  END AS trend
FROM ggsql:airquality
GROUP BY WEEKOFYEAR(Date)

VISUALISE date AS x, trend AS colour
  DRAW segment 
    MAPPING open AS y, close AS yend
    SETTING linewidth => 5
  DRAW segment 
    MAPPING low AS y, high AS yend