docs: a user guide covering getting started, architecture, embedding, data, interaction, styling and testing
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# Styling
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Every colour and metric the renderer draws with lives in `render::Style`
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(`include/plot/render/style.hpp`). A caller sets fields on `Renderer::style()`; the next frame
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picks them up, because nothing here is baked into a shader.
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Set colours and metrics on the **renderer**, not on the plot: `Renderer::draw()` copies its
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`Style` into the plot and hands it the `ui_scale` *before* it lays out, so the strips, tick
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densities and strokes the plot measures with are the values the renderer draws with. A plot
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drawn without a renderer lays out with whatever `Plot::set_style()` was given.
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## The unit rule: logical versus device
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A metric a user perceives as a **size** — a font, a stroke width, a padding, a marker, a
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swatch, the minimum spacing between ticks — is written in **logical** units, so a 12 px label
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is the same physical size on a 1x and a 2x panel. The host states its display with `ui_scale`
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(device pixels per logical unit); every consumer converts exactly once, through `device()`:
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```cpp
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f32 device(f32 logical) const noexcept {
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return ui_scale > 0.0f ? logical * ui_scale : logical; // non-positive scale converts 1:1
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}
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```
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`Plot::layout()` uses it to size the label strips, title strips and legend chrome; the renderer
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uses it to convert widths and font px (the font is reset at `device(axis_font_px)`).
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The one exception is `dense_threshold_spp`, which is a **density** (samples per *rasterised*
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pixel), compared against a ratio computed from the framebuffer's own width. Scaling it would
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make the chosen LOD level depend on the display rather than on the data, so it is used raw.
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## `Style` fields
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Colours (neither logical nor device — nothing to scale):
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| field | default | meaning |
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|---|---|---|
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| `background` | `0x101418` | frame clear colour and legend/readout panel base |
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| `grid_major` | `0x2A3038` | major (labelled-tick) grid lines |
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| `grid_minor` | `0x1C2126` | minor (subdivision) grid lines |
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| `axis_line` | `0x8A94A6` | axis lines |
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| `axis_text` | `0xC8D0DC` | tick labels and axis titles |
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| `cursor` | `0xFFB020` | cursor markers; also the hovered/grab highlight |
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| `measure_fill` | `0xFFB020` at α 0.15 | the measurement band |
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| `crosshair` | `0xE6EBF2` | crosshair lines and the readout panel's border (hue-neutral by design; trace markers keep each trace's own colour) |
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The scale:
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| field | default | unit |
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|---|---|---|
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| `ui_scale` | `1.0` | device pixels per logical unit; supplied by the host |
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Logical metrics (scaled by `device()`):
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| field | default | meaning |
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|---|---|---|
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| `axis_font_px` | `12.0` | logical em the axis text is drawn at |
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| `grid_label_pad` | `4.0` | gap between a tick label and the axes, and the inset inside a title strip |
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| `grid_major_width` | `1.0` | width of a major grid line |
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| `grid_minor_width` | `1.0` | width of a minor grid line |
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| `grid_major_visible` | `true` | stage the major grid at all |
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| `grid_minor_visible` | `true` | stage the minor grid at all |
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| `line_width_px` | `1.5` | width of the strokes framing the data (axis lines, cursor markers) |
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| `handle_width_px` | `5.0` | width of a highlighted draggable handle (a hovered/grabbed cursor tab, a region's hot bound) |
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| `tick_min_px` | `60.0` | minimum spacing between major ticks |
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Crosshair stroke and dash (logical):
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| field | default | meaning |
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|---|---|---|
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| `crosshair_width` | `0.75` | the crosshair's stroke width (deliberately thinner than a trace's `1.5`) |
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| `crosshair_dash_on` | `4.0` | ink length of the dash |
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| `crosshair_dash_off` | `4.0` | gap length of the dash; **`0` means solid** |
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Device metric (never scaled):
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| field | default | meaning |
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|---|---|---|
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| `dense_threshold_spp` | `1.5` | samples per device pixel at or above which a trace is gathered from the LOD pyramid instead of from raw samples |
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## The grid
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The grid's **positions** come from the tick set alone: `Grid::build()` emits one `GridLine`
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per tick, spanning the perpendicular axis' full range, tagged `major` (a labelled tick) or
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not. `Style` decides only how each class is drawn — its width and visibility — and the two
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colours above.
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A class switched off stages **no geometry at all**: the renderer skips it before the first
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segment is staged, so a dense plot's minor grid costs nothing rather than being drawn
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transparent. `Grid` therefore carries both classes' geometry whatever the style says; a
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caller that draws the grid itself applies the same two switches. `Grid::vertical` and
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`Grid::horizontal` (both `true`) control which directions emit lines at all.
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## The crosshair's dash
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`crosshair_dash_on`/`crosshair_dash_off` are the fragment shader's discard pattern, so no
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geometry is added. `dash_off == 0` draws a solid line — the value every other line in the
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library uses, so the one pipeline serves them unchanged. A single plot may override all three
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knobs without touching the global `Style` through `CrosshairStyle` (below).
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## `Legend`
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`Legend` (`include/plot/legend.hpp`), set with `Plot::set_legend()`. All sizes are **logical**,
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converted through `Style::device()` when `Plot::layout()` measures the panel.
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| field | default | meaning |
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|---|---|---|
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| `visible` | `true` | draw the legend at all (free when nothing is named) |
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| `place` | `LegendPlace::TopRight` | corner of `axes` the panel is anchored to |
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| `frame` | `true` | paint the panel (background at partial alpha plus a 1-px border) |
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| `pad` | `6.0` | inset from the panel edge to its first row, and the swatch↔name gutter (one knob for both) |
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| `swatch_w` | `14.0` | swatch width |
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| `swatch_h` | `8.0` | swatch height; the row height is `max(swatch_h, line height)` |
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| `row_gap` | `3.0` | blank space between rows |
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| `margin` | `8.0` | inset from the axes edge, *and* the budget the panel must fit in (so it drops entries rather than overhanging) |
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There is no "outside the plot" placement: the strips around `axes` belong to the tick labels
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and axis titles, and a legend that reserved space would resize the plot the moment a trace was
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named. A truncated legend ends with the constant `kLegendEllipsis` (`"..."`).
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## `CrosshairStyle`
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A per-`Crosshair` override of the three crosshair knobs, in logical pixels
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(`include/plot/crosshair.hpp`), set with `Plot::set_crosshair_style()`. Every field defaults to
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`-1.0`, which means "not overridden": the field falls back to the matching `Style::crosshair_*`
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metric, so a default-constructed `CrosshairStyle` is exactly the global style.
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| field | default | meaning |
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|---|---|---|
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| `width` | `-1.0` | logical stroke width; `< 0` takes `Style::crosshair_width` |
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| `dash_on` | `-1.0` | logical ink length; `< 0` takes `Style::crosshair_dash_on` |
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| `dash_off` | `-1.0` | logical gap; `< 0` takes `Style::crosshair_dash_off`, and `0` (once resolved) means solid |
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## Traces
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A trace's own `width` (`TraceDesc::width`, `Trace::width`) is **logical** too — a stroke like
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`line_width_px`, converted by the same `device()` call. Its default colour is `0x4FA3FF` and
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its default width `1.5`. Region fill/edge colours are the region's own (`Region::fill`
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`0xFFB020` at α 0.15, `Region::edge` `0xFFB020`); a cursor's colour is per-cursor
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(`0xFFB020` by default).
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## Installing a text backend
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The font a plot measures with and the renderer draws with is a `render::TextBackend`
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(`include/plot/render/text.hpp`): `measure()`, `height()`, `ascent()`, `em_px()`, `glyph()`,
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`cell_uv()`, `atlas_pixels()`, `atlas_width()`, `atlas_height()` and `revision()`.
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- `Plot::set_text_backend(const render::TextBackend*)` sets the backend a plot lays its labels
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out with. It is **borrowed**, not owned, and may be null (then the label areas fall back to
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`PlotLayout`'s defaults and no string is measured).
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- `Renderer::set_text_backend(const render::TextBackend*)` sets the face the renderer draws
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with. It is also borrowed and must outlive the frames.
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**Note the interaction:** `Renderer::init()` binds `render::EmbeddedMonoFont`, and every
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`Renderer::draw()` installs that binding on the plot it is given — replacing whatever a host
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set with `Plot::set_text_backend()` (a debug build names the one it dropped). So installing a
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backend on the *plot* only takes effect for a plot no renderer draws. To draw with your own
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face (say a `plot::text::TrueTypeFont` from the optional `plot::font_ttf` target), install it
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with `Renderer::set_text_backend()`; the swap takes effect on the next frame, which re-reads
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the backend's metrics at `style().device(axis_font_px)` and re-uploads its atlas when the
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backend's `revision()` moved. Passing `nullptr` restores the embedded face.
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