theme

type Option func(*Theme)

type Theme struct {
	Name string

	// Surfaces.
	Background ir.Color // outside the plot area
	PlotFill   ir.Color // inside the plot area

	// Text.
	FontFamily string
	TitleSize  float64
	LabelSize  float64
	TickSize   float64
	TitleColor ir.Color
	LabelColor ir.Color
	TickColor  ir.Color

	// Axes and grid.
	AxisColor      ir.Color
	AxisWidth      float32
	GridColor      ir.Color
	GridWidth      float32
	GridDash       []float32
	TickLength     float32
	TickLabelPad   float32
	AxisTitlePad   float32
	ShowGridX      bool
	ShowGridY      bool
	ShowAxisLineX  bool
	ShowAxisLineY  bool
	TickCountHintX int
	TickCountHintY int

	// Legend.
	LegendSwatch  float32
	LegendPad     float32
	LegendGap     float32
	LegendColor   ir.Color
	LegendBG      ir.Color
	LegendBorder  ir.Color
	LegendPadding float32

	// Guides. A chart can carry more than one — a legend for the layers that
	// have a colour each, a colourbar for a layer whose colour comes from a
	// continuous scale — and they stack in one column beside the plot.
	GuideGap float32 // vertical gap between stacked guides

	// Colourbar.
	ColorbarThickness float32 // across the bar
	ColorbarFraction  float64 // of the plot height, before clamping
	ColorbarBorder    ir.Color
	ColorbarTickCount int

	// Annotations: reference lines, bands and labels that are not data.
	AnnotationColor   ir.Color
	AnnotationWidth   float32
	AnnotationDash    []float32
	AnnotationOpacity float64 // fill alpha for a shaded band

	// Panels: subplots and facets.
	PanelGap    float32 // between adjacent panels
	StripBG     ir.Color
	StripColor  ir.Color
	StripSize   float64
	StripPad    float32
	StripBorder ir.Color

	// Spacing.
	Margin float32 // outer margin on all four sides

	// Series colours, and the ramps a colour scale falls back to.
	Palette    palette.Qualitative
	Sequential palette.Ramp
	Diverging  palette.Ramp

	// Default geometry weights.
	LineWidth  float32
	MarkerSize float32
}

type Tokens struct {
	// Name identifies the theme, and is the key it registers under.
	Name string

	// Background is the canvas outside the plot area; Panel is the fill
	// inside it, normally transparent so the background shows through; Strip
	// is the band a facet's label sits in.
	Background ir.Color
	Panel      ir.Color
	Strip      ir.Color

	// Ink runs from the strongest text to the faintest: a title, a label, a
	// tick. Three levels is enough to build a hierarchy and few enough that a
	// second theme has to answer only three questions.
	Ink       ir.Color
	InkMuted  ir.Color
	InkSubtle ir.Color

	// Line is the axis; LineSubtle is the grid behind the data.
	Line       ir.Color
	LineSubtle ir.Color

	// FontFamily is the logical family; "" means the backend's default sans.
	FontFamily string

	// FontSize is the label size in device units. The title and tick sizes
	// derive from it — see [Build].
	FontSize float64

	// Space is the spacing unit every padding and gap is a multiple of.
	Space float64

	// Palette colours the layers of a chart; Sequential and Diverging are the
	// ramps a colour scale falls back to when it is given none.
	Palette    palette.Qualitative
	Sequential palette.Ramp
	Diverging  palette.Ramp
}