Patterns and Hatching

Hatching is how BLAND replaces color. Every filled region — bar, area — takes a pattern instead of a fill color. Patterns stay distinguishable after photocopy, on grayscale printers, and for readers with color vision deficiency.

The preset gallery

Bland.Patterns.preset_cycle/0 returns these in the order the renderer walks when a series doesn't name a :hatch explicitly. The ordering is tuned for maximum visual separation at each step: light hatches alternate with darker ones, orientation alternates too, and solid-fill presets sit at the ends of the list.

Usage

Pick by name:

Bland.bar(fig, ["A", "B", "C"], [3, 5, 2], hatch: :crosshatch, label: "set 1")

Or let the renderer cycle:

# No :hatch given → :solid_white, then :diagonal, then :anti_diagonal, …
fig
|> Bland.bar(cats, run_a, label: "A")
|> Bland.bar(cats, run_b, label: "B")
|> Bland.bar(cats, run_c, label: "C")

Choosing patterns

Some practical guidance, gathered from cartography and technical drafting conventions:

• Lighter patterns (dots, sparse lines) for larger regions. A big bar with :crosshatch will visually dominate everything else on the plot.
• Orient hatching differently for adjacent regions. :diagonal next to :anti_diagonal reads clearly even in a greyscale photo­copy.
• Reserve solid fills for emphasis. Solid black draws the eye immediately; use it for the one series you want the reader to see first.
• Keep stroke weight consistent across fills. The default pattern library draws at 1 px, matching BLAND's default series stroke weight — change one and you should change the other.

Custom patterns

Call Bland.Patterns.define/4 with a DOM id, tile size, and SVG body:

# A pattern of tiny horizontal tick marks
Bland.Patterns.define("my-ticks", {6, 6},
  ~s|<line x1="0" y1="3" x2="6" y2="3" stroke="black" stroke-width="1"/>|
)

To use a custom pattern, insert the pattern's <defs> into the generated SVG yourself and reference it via fill="url(#my-ticks)". The high-level :hatch option on Bland.bar/4 expects a preset atom, so custom patterns are an "eject into raw SVG" escape hatch.

If you find yourself reaching for the escape hatch often, open an issue — the preset set is meant to grow.

Stroke dashes

Lines don't have a fill, but they still need to be distinguishable. BLAND ships six dash presets, cycled by Bland.Strokes.preset_cycle/0:

:dash_dot is the classic centerline / phantom line from mechanical drafting — useful for reference traces that should visually sit behind the primary curves.

Override with a raw SVG dasharray if you need something off-preset:

Bland.line(fig, xs, ys, stroke: "4 2 1 2")

Markers

Scatter and marked lines pull from Bland.Markers.preset_cycle/0:

:circle_open, :square_open, :triangle_open, :diamond_open, :cross, :plus, :circle_filled, :square_filled, :triangle_filled, :diamond_filled, :asterisk, :dot.

The ordering is alternating-open-then-filled so two adjacent series look distinct by shape and fill.

Bland.scatter(fig, xs, ys,
  marker: :triangle_filled,
  marker_size: 5,
  label: "trial 3"
)

A full reference example

cats = ["Q1", "Q2", "Q3", "Q4"]

fig =
  Bland.figure(size: :a5_landscape, title: "Pattern reference")
  |> Bland.axes(ylabel: "count")
  |> Bland.bar(cats, [10, 12, 15, 11], label: ":diagonal",      hatch: :diagonal,      group: 1)
  |> Bland.bar(cats, [8,  14, 16, 13], label: ":crosshatch",    hatch: :crosshatch,    group: 2)
  |> Bland.bar(cats, [12, 11, 14, 17], label: ":dots_sparse",   hatch: :dots_sparse,   group: 3)
  |> Bland.bar(cats, [9,  13, 12, 15], label: ":horizontal",    hatch: :horizontal,    group: 4)
  |> Bland.legend(position: :top_left, title: "Fill")

Bland.write!(fig, "patterns.svg")