fix(render): convert Trace::width through the UI scale, and fold the controller's tolerance conversion into Style::device()

include/plot/input/controller.hpp

@@ -59,9 +59,11 @@ enum class DragMode {
 /// therefore change its *physical* size with the display: a 5 px grab radius is
 /// half the hand movement on a 2x panel, so the same cursor becomes harder to
 /// catch there. Both tolerances -- the cursor grab radius and the box-zoom click
-/// threshold -- are therefore logical constants multiplied by `Plot::ui_scale()`,
-/// which is also why the controller reads the scale off the plot it already holds
-/// instead of keeping a copy a host would have to keep in step.
+/// threshold -- are therefore logical constants, converted through
+/// `Style::device()` at the scale of the plot the controller already holds: that is
+/// the single place the logical -> device rule lives, and the plot is the one that
+/// knows what its device pixel is worth, so the controller keeps no copy of the scale
+/// for a host to keep in step.
 ///
 /// The middle-drag axis zoom's per-pixel gain is deliberately *not* in that group:
 /// it is a data-navigation sensitivity rather than a size (the wheel and `+`/`-`

include/plot/plot/trace.hpp

@@ -11,6 +11,9 @@ namespace plot {
 struct TraceDesc {
   const data::Series* series = nullptr;
   Color color = Color::hex(0x4FA3FF);
+  /// The stroke's width in **logical** pixels, like every stroke in `Style` (see
+  /// that struct's unit rule): the renderer converts it once, so a trace has the
+  /// same weight against the axis lines whatever the display's DPI.
   f32 width = 1.5f;
   u32 axis_x = 0, axis_y = 0;
   /// What the trace is called, for the legend. Empty means "do not list it", which
@@ -28,6 +31,8 @@ struct TraceDesc {
 struct Trace {
   const data::Series* series = nullptr;
   Color color = Color::hex(0x4FA3FF);
+  /// Logical pixels, as in `TraceDesc::width`: converted to device pixels by the
+  /// renderer's one `Style::device()` call per stroke.
   f32 width = 1.5f;
   u32 axis_x = 0, axis_y = 0;
   /// The legend's entry for this trace, empty when it has none. A name does not

include/plot/render/style.hpp

@@ -24,8 +24,13 @@ namespace plot::render {
 /// chosen LOD level depend on the display and not on the data. It is used raw.
 ///
 /// The same rule classifies the metrics that live outside this struct, which are
-/// logical too and converted by the same call: `Legend`'s pad, swatch, gap and
-/// margin, and the renderer's marker arm lengths and text offsets.
+/// logical too and converted by the same call:
+///
+///   * `Legend`'s pad, swatch, gap and margin;
+///   * `Trace`'s (and `TraceDesc`'s) `width` -- a stroke like `line_width_px`, and
+///     the one a plot draws most of, so leaving it raw would invert the
+///     trace/axis stroke hierarchy at any scale above 1;
+///   * the renderer's marker arm lengths and text offsets.
 struct Style {
   // --- colours: neither logical nor device, so nothing to scale ------------