plotter/tests/test_figure.cpp

// `Figure`: several plots stacked in one column, sharing one x range.
//
// The layout cases pin the geometry the renderer draws with -- the row rects, the
// weights, the floor and the shared strip -- and the link cases pin the propagation
// contract: one range for a whole group, applied to every member in one pass, and a
// group that follows deriving one window from the newest sample across all of its
// members rather than one per row.
#include "harness.hpp"

#include <plot/data/array_series.hpp>
#include <plot/data/stream_series.hpp>
#include <plot/plot/figure.hpp>

#include <cmath>
#include <vector>

namespace {

constexpr plot::f32 kW = 640.0f;
constexpr plot::f32 kH = 480.0f;

plot::Rect whole_figure() { return plot::Rect{0.0f, 0.0f, kW, kH}; }

/// A figure whose every row has a y axis and an x axis, so a row is a plot in the
/// shape the layout tests care about: it loses width to a y strip and, on the bottom
/// row alone, height to the shared x strip -- which lives inside that row rather than
/// in a band of its own.
plot::Figure figure_with_axes(plot::u32 rows) {
  plot::Figure figure(rows);
  figure.set_viewport(whole_figure());
  for (plot::u32 row = 0; row < rows; ++row) {
    plot::Plot& p = figure.plot(row);
    p.add_x_axis({"t", {0.0, 100.0}, false, plot::AxisPlace::Bottom, false});
    p.add_y_axis({"v", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
  }
  return figure;
}

/// The rows of a laid-out figure, so a case can assert on them without repeating the
/// call that produces them.
std::vector<plot::Rect> rows_of(plot::Figure& figure) {
  return figure.layout().rows;
}

bool rows_overlap(const plot::Rect& a, const plot::Rect& b) {
  return a.x < b.right() && b.x < a.right() && a.y < b.bottom() && b.y < a.bottom();
}

}  // namespace

// ------------------------------------------------------------------- layout

/// The rows tile the figure: they cover it from top to bottom without a gap and
/// without two of them overlapping, which is what makes the shared strip's position
/// -- and every row's axes -- a statement rather than a hope.
PLOT_TEST(row_rects_tile_the_figure_without_overlap) {
  plot::Figure figure = figure_with_axes(3u);
  const std::vector<plot::Rect> rows = rows_of(figure);
  PLOT_CHECK(rows.size() == 3u);

  // The figure's width, straight through: a row's own y strips are inside its rect,
  // not carved out of the figure.
  for (const plot::Rect& row : rows) {
    PLOT_CHECK_NEAR(static_cast<plot::f64>(row.x), 0.0, 1e-6);
    PLOT_CHECK_NEAR(static_cast<plot::f64>(row.w), static_cast<plot::f64>(kW), 1e-6);
    PLOT_CHECK(row.h > 0.0f);
  }
  // Top to bottom, in row order, touching: each row starts where the one above it
  // ended, and the last one ends on the figure's bottom edge -- the rows tile the
  // whole figure, because the shared x axis is the bottom row's own rather than a band
  // taken off the rows.
  const plot::FigureLayout layout = figure.layout();
  PLOT_CHECK_NEAR(static_cast<plot::f64>(rows[0].y), 0.0, 1e-6);
  for (std::size_t i = 1; i < rows.size(); ++i) {
    PLOT_CHECK_NEAR(static_cast<plot::f64>(rows[i].y), static_cast<plot::f64>(rows[i - 1].bottom()),
                    1e-6);
  }
  PLOT_CHECK_NEAR(static_cast<plot::f64>(rows.back().bottom()), static_cast<plot::f64>(kH), 1e-6);
  // The shared strip *is* the bottom row's own x label area: inside that row, below the
  // row's axes and above the row's title strip, and as wide as its axes -- which is what
  // puts it where the controller can hit-test it and where the renderer stages the one
  // set of labels.
  const plot::PlotLayout& bottom = figure.plot(2u).layout();
  PLOT_CHECK(layout.x_axis_strip.h > 0.0f);
  PLOT_CHECK(layout.x_axis_strip.y >= layout.rows.back().y);
  PLOT_CHECK(layout.x_axis_strip.bottom() <= layout.rows.back().bottom() + 1e-6f);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(layout.x_axis_strip.h),
                  static_cast<plot::f64>(bottom.x_label_area.h), 1e-6);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(layout.x_axis_strip.x),
                  static_cast<plot::f64>(bottom.x_label_area.x), 1e-6);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(layout.x_axis_strip.y),
                  static_cast<plot::f64>(bottom.x_label_area.y), 1e-6);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(layout.x_axis_strip.w),
                  static_cast<plot::f64>(bottom.x_label_area.w), 1e-6);

  for (std::size_t i = 0; i < rows.size(); ++i) {
    for (std::size_t j = i + 1; j < rows.size(); ++j) {
      PLOT_CHECK(!rows_overlap(rows[i], rows[j]));
    }
  }

  // A gap keeps the rows apart by exactly that much and is not overlap by another
  // name: the rows cover the region between the strip and the top, minus the gaps.
  figure.set_row_gap(10.0f);
  const std::vector<plot::Rect> spaced = rows_of(figure);
  PLOT_CHECK(spaced.size() == 3u);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(spaced[1].y - spaced[0].bottom()), 10.0, 1e-6);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(spaced[2].y - spaced[1].bottom()), 10.0, 1e-6);
  PLOT_CHECK(spaced[0].h < rows[0].h);  // the gaps came out of the rows
  // The last spaced row still ends on the figure's bottom edge -- the gaps came out of
  // the rows' share of the figure, not out of the figure -- and so does the strip.
  PLOT_CHECK_NEAR(static_cast<plot::f64>(spaced.back().bottom()), static_cast<plot::f64>(kH), 1e-6);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.layout().x_axis_strip.bottom()),
                  static_cast<plot::f64>(kH), 1e-6);

  // A figure with no rows has no geometry -- and no row to reach for while computing
  // it, which is the case that would read past the end of its own plot list.
  plot::Figure empty(0u);
  empty.set_viewport(whole_figure());
  PLOT_CHECK(empty.row_count() == 0u);
  PLOT_CHECK(empty.layout().rows.empty());
  PLOT_CHECK(empty.layout().x_axis_strip.h == 0.0f);
  PLOT_CHECK(empty.layout().row_gap == 0.0f);
}

/// Weights change how the figure's height is divided -- a taller main plot, shorter
/// rows under it -- and the rows still account for all of it: the sum of the heights
/// plus the gaps is the whole figure (the shared strip lives inside the bottom row, so
/// it costs the division nothing), so no weight can leave a band unclaimed or push a
/// row past the bottom.
PLOT_TEST(row_weights_change_relative_heights_and_sum_to_the_viewport) {
  plot::Figure figure = figure_with_axes(3u);
  const std::vector<plot::Rect> equal = rows_of(figure);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(equal[0].h), static_cast<plot::f64>(equal[1].h), 1e-4);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(equal[1].h), static_cast<plot::f64>(equal[2].h), 1e-4);

  const float main_taller[] = {3.0f, 1.0f, 1.0f};
  plot::Status st = figure.set_row_weights(main_taller);
  PLOT_CHECK(st);
  PLOT_CHECK(figure.has_row_weights());
  // By value, not a reference into the figure: a later `layout()` rewrites the
  // figure's own vectors in place, and a case that kept pointing at them would
  // compare a division with itself.
  const plot::FigureLayout layout = figure.layout();
  const std::vector<plot::Rect> rows = layout.rows;
  PLOT_CHECK(rows[0].h > rows[1].h);
  PLOT_CHECK_NEAR(static_cast<plot::f64>(rows[1].h), static_cast<plot::f64>(rows[2].h), 1e-4);
  // Monotone in the weights: a bigger share is a taller row, and what the taller row
  // gained the others lost -- the figure's height is divided, never added to. Every
  // row keeps the floor, so a weight is a share of what is left above it and the
  // *order* is the exact statement, not the ratio.
  const float main_taller_still[] = {6.0f, 1.0f, 1.0f};
  PLOT_CHECK(figure.set_row_weights(main_taller_still));
  const std::vector<plot::Rect> taller = rows_of(figure);
  PLOT_CHECK(taller[0].h > rows[0].h);
  PLOT_CHECK(taller[1].h < rows[1].h);
  PLOT_CHECK(taller[0].h + taller[1].h + taller[2].h < rows[0].h + rows[1].h + rows[2].h + 1e-3f);
  PLOT_CHECK(figure.set_row_weights(main_taller));
  PLOT_CHECK_NEAR(static_cast<plot::f64>(figure.layout().rows[0].h),
                  static_cast<plot::f64>(rows[0].h), 1e-4);

  plot::f32 total = 0.0f;
  for (const plot::Rect& row : rows) total += row.h;
  total += layout.row_gap * 2.0f;
  PLOT_CHECK_NEAR(static_cast<plot::f64>(total), static_cast<plot::f64>(kH), 1e-3);

  // Any positive scale is the same division: normalised, not absolute.
  const float same_ratio[] = {30.0f, 10.0f, 10.0f};
  st = figure.set_row_weights(same_ratio);
  PLOT_CHECK(st);
  const std::vector<plot::Rect> scaled = rows_of(figure);
  for (std::size_t i = 0; i < rows.size(); ++i) {
    PLOT_CHECK_NEAR(static_cast<plot::f64>(scaled[i].h), static_cast<plot::f64>(rows[i].h), 1e-4);
  }

  // Back to an equal division.
  figure.clear_row_weights();
  PLOT_CHECK(!figure.has_row_weights());
  const std::vector<plot::Rect> again = rows_of(figure);
  for (std::size_t i = 0; i < again.size(); ++i) {
    PLOT_CHECK_NEAR(static_cast<plot::f64>(again[i].h), static_cast<plot::f64>(equal[i].h), 1e-4);
  }
}

/// A weight set that cannot divide the figure is refused outright -- a row with no
/// share has no rect to give it, and a weight the arithmetic cannot use would produce
/// exactly that -- and the figure keeps the division it had.
PLOT_TEST(degenerate_row_weights_are_rejected) {
  plot::Figure figure = figure_with_axes(3u);
  const std::vector<plot::Rect> before = rows_of(figure);

  const float zero[] = {1.0f, 0.0f, 1.0f};
  PLOT_CHECK(!figure.set_row_weights(zero));
  const float negative[] = {1.0f, -2.0f, 1.0f};
  PLOT_CHECK(!figure.set_row_weights(negative));
  const float none[] = {0.0f, 0.0f, 0.0f};
  PLOT_CHECK(!figure.set_row_weights(none));
  const float nan_weight[] = {1.0f, std::nanf(""), 1.0f};
  PLOT_CHECK(!figure.set_row_weights(nan_weight));
  const float inf_weight[] = {1.0f, INFINITY, 1.0f};
  PLOT_CHECK(!figure.set_row_weights(inf_weight));
  // The wrong count is not a division of *these* rows either.
  const float too_few[] = {1.0f, 1.0f};
  PLOT_CHECK(!figure.set_row_weights(too_few));
  const float too_many[] = {1.0f, 1.0f, 1.0f, 1.0f};
  PLOT_CHECK(!figure.set_row_weights(too_many));

  PLOT_CHECK(!figure.has_row_weights());
  const std::vector<plot::Rect> after = rows_of(figure);
  PLOT_CHECK(after.size() == before.size());
  for (std::size_t i = 0; i < after.size(); ++i) {
    PLOT_CHECK_NEAR(static_cast<plot::f64>(after[i].h), static_cast<plot::f64>(before[i].h), 1e-6);
  }

  // A refused set is refused *before* it is stored, so one good set after six bad
  // ones is what the figure divides by.
  const float good[] = {2.0f, 1.0f, 1.0f};
  PLOT_CHECK(figure.set_row_weights(good));
  const std::vector<plot::Rect> weighted = rows_of(figure);
  PLOT_CHECK(weighted[0].h > weighted[1].h);
}

/// Every row keeps a positive height, whatever the figure's size and whatever the
/// weights say -- a row that is not there is a plot that is not there, silently. The
/// shared strip cannot take a row's height for the same reason: it is the bottom row's
/// own, so chrome may shrink one row's *axes*, never erase a row.
PLOT_TEST(every_row_keeps_a_positive_height) {
  // Five rows in a figure shorter than the strip one row would ask for.
  plot::Figure tiny(5u);
  tiny.set_viewport({0.0f, 0.0f, 200.0f, 12.0f});
  tiny.set_row_gap(6.0f);  // more gap than the figure has height
  const plot::FigureLayout layout = tiny.layout();
  PLOT_CHECK(layout.rows.size() == 5u);
  plot::f32 total = 0.0f;
  for (const plot::Rect& row : layout.rows) {
    PLOT_CHECK(row.h > 0.0f);
    total += row.h;
  }
  // The rows took the whole figure: a gap that does not fit is dropped, and the shared
  // strip is inside the bottom row rather than a band beside it, so it costs the
  // division nothing.
  PLOT_CHECK_NEAR(static_cast<plot::f64>(total), 12.0, 1e-3);

  // A wildly unbalanced weight set still cannot squeeze a row out. The tiny weights
  // here are small enough that a row's *share* of the leftover underflows to zero
  // once it is expressed as a float -- which is exactly the share that would have been
  // its whole height without a floor under it.
  plot::Figure lopsided(4u);
  lopsided.set_viewport({0.0f, 50.0f, 400.0f, 300.0f});
  const float lopsided_weights[] = {1.0f, 1e-45f, 1.0f, 1e-45f};
  PLOT_CHECK(lopsided.set_row_weights(lopsided_weights));
  for (const plot::Rect& row : lopsided.layout().rows) {
    PLOT_CHECK(row.h > 0.0f);
  }
}

// --------------------------------------------------------------- the x link

/// Linking a row to a group that already has a range puts it on that range at once,
/// not on the next frame: the group's window is the group's, and a member is on it
/// from the call that made it a member.
PLOT_TEST(link_x_applies_the_group_range_immediately) {
  plot::Figure figure = figure_with_axes(3u);
  PLOT_CHECK(figure.x_link(0u) == 0u);  // every row starts unlinked
  PLOT_CHECK(figure.x_link(1u) == 0u);
  const plot::u32 group = figure.add_x_link();
  PLOT_CHECK(group == 1u);  // 0 is the unlinked group, so ids start at 1
  PLOT_CHECK(figure.add_x_link() == 2u);
  PLOT_CHECK(figure.x_link_count() == 2u);

  PLOT_CHECK(figure.link_x(0u, group));
  PLOT_CHECK(figure.x_link(0u) == group);
  // The first row into an empty group frames it, so the group is not a range the
  // caller never chose.
  PLOT_CHECK_NEAR(figure.x_link_range(group).min, 0.0, 1e-12);
  PLOT_CHECK_NEAR(figure.x_link_range(group).max, 100.0, 1e-12);

  PLOT_CHECK(figure.set_x_link_range(group, {10.0, 20.0}));
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, 10.0, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).max, 20.0, 1e-12);

  // The row joining now lands on the group's window rather than keeping its own.
  figure.plot(1u).set_x_range(0u, {500.0, 900.0});
  PLOT_CHECK(figure.link_x(1u, group));
  PLOT_CHECK_NEAR(figure.plot(1u).x_range(0u).min, 10.0, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(1u).x_range(0u).max, 20.0, 1e-12);

  // Group 0 is the way back out, and a row that leaves keeps the range it holds.
  PLOT_CHECK(figure.link_x(1u, 0u));
  PLOT_CHECK(figure.x_link(1u) == 0u);
  PLOT_CHECK_NEAR(figure.plot(1u).x_range(0u).min, 10.0, 1e-12);

  // A row or a group that does not exist is refused, and nothing moves.
  PLOT_CHECK(!figure.link_x(9u, group));
  PLOT_CHECK(!figure.link_x(0u, 9u));
  PLOT_CHECK(!figure.set_x_link_range(0u, {1.0, 2.0}));   // group 0 has no shared range
  PLOT_CHECK(!figure.set_x_link_range(9u, {1.0, 2.0}));
  PLOT_CHECK(!figure.set_x_link_follow(0u, plot::XFollow::Live));
  PLOT_CHECK(figure.x_link(0u) == group);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, 10.0, 1e-12);

  // A row with no x axis has nothing to link, and linking it must not pretend
  // otherwise.
  plot::Figure bare(1u);
  const plot::u32 g = bare.add_x_link();
  PLOT_CHECK(!bare.link_x(0u, g));
  PLOT_CHECK(bare.x_link(0u) == 0u);
}

/// A group owns one range, so the range any single member is given becomes every
/// member's before the next frame -- whether it arrived from a caller or from a
/// controller, which reach a member exactly the same way.
PLOT_TEST(setting_a_range_on_any_member_propagates_to_all) {
  plot::Figure figure = figure_with_axes(3u);
  const plot::u32 group = figure.add_x_link();
  for (plot::u32 row = 0; row < 3u; ++row) PLOT_CHECK(figure.link_x(row, group));
  PLOT_CHECK(figure.set_x_link_range(group, {5.0, 15.0}));

  // Set on each member in turn: each one is the group's range from the next layout,
  // and every other member holds it too.
  const double values[][2] = {{100.0, 200.0}, {-40.0, -10.0}, {0.5, 0.75}};
  for (plot::u32 row = 0; row < 3u; ++row) {
    figure.plot(row).set_x_range(0u, {values[row][0], values[row][1]});
    figure.layout();
    for (plot::u32 member = 0; member < 3u; ++member) {
      PLOT_CHECK_NEAR(figure.plot(member).x_range(0u).min, values[row][0], 1e-12);
      PLOT_CHECK_NEAR(figure.plot(member).x_range(0u).max, values[row][1], 1e-12);
    }
    PLOT_CHECK_NEAR(figure.x_link_range(group).min, values[row][0], 1e-12);
    PLOT_CHECK_NEAR(figure.x_link_range(group).max, values[row][1], 1e-12);
  }

  // A member with auto scale armed and data of its own cannot drag the group back:
  // the push applies the group's range, and applying a range is what clears auto
  // scale -- so a linked row is never re-fitted out of the group's window, however
  // many frames go by. The trace's own bounds are a thousand wide, so a row that
  // fitted itself would be unmistakable.
  plot::data::ArraySeries wide(std::vector<plot::f32>(64u, 0.5f), 1000.0, 1.0);
  figure.plot(2u).add_trace({&wide, plot::Color::hex(0x40C0FF), 1.5f, 0u, 0u});
  figure.plot(2u).set_x_auto_scale(0u, true);
  PLOT_CHECK(figure.set_x_link_range(group, {1.0, 2.0}));
  for (int frame = 0; frame < 3; ++frame) {
    figure.layout();
    PLOT_CHECK_NEAR(figure.plot(2u).x_range(0u).min, 1.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(2u).x_range(0u).max, 2.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, 1.0, 1e-12);
  }
}

/// The propagation settles in one pass and stays settled: after a frame every member
/// holds the group's range, and a member written back with a range that *is* the
/// group's -- the write a re-propagating implementation would read as a change -- does
/// not move it again. This is the case that catches a write-back loop: a design that
/// compared a member against the group's range instead of against what it pushed would
/// re-derive the group's state from its own output on every frame.
PLOT_TEST(propagation_is_idempotent_and_terminates) {
  plot::Figure figure = figure_with_axes(2u);
  const plot::u32 group = figure.add_x_link();
  PLOT_CHECK(figure.link_x(0u, group));
  PLOT_CHECK(figure.link_x(1u, group));
  PLOT_CHECK(figure.set_x_link_range(group, {0.0, 10.0}));

  // A long alternating sequence: a change on one row, one layout, and both rows
  // agree. "One pass" is exactly this -- a second layout with no new change moves
  // nothing, so the frame after a change is the frame that settles it.
  for (plot::u32 step = 0; step < 500u; ++step) {
    const plot::u32 row = step % 2u;
    const double lo = static_cast<double>(step) * 0.25;
    figure.plot(row).set_x_range(0u, {lo, lo + 3.0});
    figure.layout();
    for (plot::u32 member = 0; member < 2u; ++member) {
      PLOT_CHECK_NEAR(figure.plot(member).x_range(0u).min, lo, 1e-9);
      PLOT_CHECK_NEAR(figure.plot(member).x_range(0u).max, lo + 3.0, 1e-9);
    }
    // Nothing left to propagate: the members already hold what the group holds, so
    // further frames are the identity on the range.
    const plot::Range settled = figure.x_link_range(group);
    for (int frame = 0; frame < 3; ++frame) {
      figure.layout();
      PLOT_CHECK_NEAR(figure.x_link_range(group).min, settled.min, 1e-12);
      PLOT_CHECK_NEAR(figure.x_link_range(group).max, settled.max, 1e-12);
      for (plot::u32 member = 0; member < 2u; ++member) {
        PLOT_CHECK_NEAR(figure.plot(member).x_range(0u).min, settled.min, 1e-12);
        PLOT_CHECK_NEAR(figure.plot(member).x_range(0u).max, settled.max, 1e-12);
      }
    }
  }

  // ... and deliberately written back in: a member handed the group's own range --
  // what a propagated range is -- must not be taken for a new change and pushed
  // again, in a frame or in a hundred.
  const plot::Range held = figure.x_link_range(group);
  for (int frame = 0; frame < 100; ++frame) {
    figure.plot(1u).set_x_range(0u, held);
    figure.layout();
    PLOT_CHECK_NEAR(figure.x_link_range(group).min, held.min, 1e-12);
    PLOT_CHECK_NEAR(figure.x_link_range(group).max, held.max, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, held.min, 1e-12);
  }
}

/// Rows that are not in a group are not in one: each keeps whatever its own plot was
/// given, however many frames go by, and a group's window is nobody else's business.
PLOT_TEST(unlinked_plots_keep_independent_ranges) {
  plot::Figure figure = figure_with_axes(3u);
  figure.plot(0u).set_x_range(0u, {1.0, 2.0});
  figure.plot(1u).set_x_range(0u, {10.0, 40.0});
  figure.plot(2u).set_x_range(0u, {-5.0, 5.0});

  // A group elsewhere in the figure, to prove it is the group that shares and not
  // the figure: adding one changes no row.
  const plot::u32 group = figure.add_x_link();
  PLOT_CHECK(figure.x_link_count() == 1u);

  for (int frame = 0; frame < 5; ++frame) {
    figure.layout();
    PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, 1.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).max, 2.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(1u).x_range(0u).min, 10.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(1u).x_range(0u).max, 40.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(2u).x_range(0u).min, -5.0, 1e-12);
    PLOT_CHECK_NEAR(figure.plot(2u).x_range(0u).max, 5.0, 1e-12);
  }
  PLOT_CHECK(figure.x_link(0u) == 0u && figure.x_link(1u) == 0u && figure.x_link(2u) == 0u);

  // Linking one row does not touch the others.
  PLOT_CHECK(figure.link_x(1u, group));
  PLOT_CHECK_NEAR(figure.x_link_range(group).min, 10.0, 1e-12);
  PLOT_CHECK_NEAR(figure.x_link_range(group).max, 40.0, 1e-12);
  figure.layout();
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, 1.0, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).max, 2.0, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(2u).x_range(0u).min, -5.0, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(2u).x_range(0u).max, 5.0, 1e-12);
}

/// A `Live` axis owns its own range -- its own span, its own offset, its own live
/// edge -- so a link group cannot own it too. Linking one is refused with a `Status`
/// rather than silently reinterpreted: the caller is told, and the axis is left
/// exactly as it was.
PLOT_TEST(linking_a_live_axis_is_refused_with_a_status) {
  plot::Figure figure = figure_with_axes(2u);
  const plot::u32 group = figure.add_x_link();
  figure.plot(0u).set_x_follow(0u, plot::XFollow::Live);
  PLOT_CHECK(figure.plot(0u).x_follow(0u) == plot::XFollow::Live);
  const plot::Range before = figure.plot(0u).x_range(0u);

  const plot::Status st = figure.link_x(0u, group);
  PLOT_CHECK(!st);
  PLOT_CHECK(st.code == plot::ErrorCode::InvalidArgument);
  PLOT_CHECK(st.message[0] != '\0');  // a refusal a caller can read
  PLOT_CHECK(figure.x_link(0u) == 0u);
  // ... and it is still Live, with the range it had: a refusal that changed the
  // state would be the silent re-interpretation it refuses to be.
  PLOT_CHECK(figure.plot(0u).x_follow(0u) == plot::XFollow::Live);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, before.min, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).max, before.max, 1e-12);

  // The other row links, and the group's window leaves the live axis alone.
  PLOT_CHECK(figure.link_x(1u, group));
  PLOT_CHECK(figure.set_x_link_range(group, {3.0, 4.0}));
  figure.layout();
  PLOT_CHECK_NEAR(figure.plot(1u).x_range(0u).min, 3.0, 1e-12);
  PLOT_CHECK(figure.plot(0u).x_follow(0u) == plot::XFollow::Live);

  // Turning the follow off is what makes the axis linkable -- the group's state is
  // the group's, so the axis has to be `Off` first.
  figure.plot(0u).set_x_follow(0u, plot::XFollow::Off);
  PLOT_CHECK(figure.link_x(0u, group));
  PLOT_CHECK(figure.x_link(0u) == group);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).min, 3.0, 1e-12);
  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).max, 4.0, 1e-12);
}

/// A following group advances every row together: the window is derived once per
/// frame from the newest x across the members' traces, exactly as one `Live` axis'
/// would be, so a streaming multi-row figure has one window and no per-row drift --
/// each row sees the same edge even though only one of them is being pushed to. The
/// two rows hold *different* amounts of data, so "the group's edge" and "this row's
/// edge" are different numbers and the assertions can tell them apart.
PLOT_TEST(a_following_group_advances_all_rows_together) {
  plot::data::StreamSeries left(1024u, 0.0, 1.0);
  plot::data::StreamSeries right(1024u, 0.0, 1.0);
  for (int i = 0; i < 300; ++i) {
    left.push(static_cast<plot::f32>(std::sin(i * 0.1)));
  }
  for (int i = 0; i < 200; ++i) {
    right.push(static_cast<plot::f32>(std::cos(i * 0.1)));
  }
  left.acquire();
  right.acquire();

  plot::Figure figure = figure_with_axes(2u);
  figure.plot(0u).add_trace({&left, plot::Color::hex(0x4FA3FF), 1.5f, 0u, 0u});
  figure.plot(1u).add_trace({&right, plot::Color::hex(0xFF8080), 1.5f, 0u, 0u});

  const plot::u32 group = figure.add_x_link();
  PLOT_CHECK(figure.link_x(0u, group));
  PLOT_CHECK(figure.link_x(1u, group));
  PLOT_CHECK(figure.set_x_link_follow(group, plot::XFollow::Live));
  PLOT_CHECK(figure.x_link_follow(group) == plot::XFollow::Live);
  // Under `Live` a range is a span and an offset behind the live edge, not a pair of
  // positions, so the group is told "ten wide, ending at the edge" -- the speed row's
  // newest x, which is where the group's window will stay.
  const plot::f64 edge0 = left.x_bounds().max;
  PLOT_CHECK(figure.set_x_link_range(group, {edge0 - 10.0, edge0}));

  const auto both_rows_hold = [&](plot::Range& out) {
    const plot::Range a = figure.plot(0u).x_range(0u);
    const plot::Range b = figure.plot(1u).x_range(0u);
    PLOT_CHECK_NEAR(a.min, b.min, 1e-9);
    PLOT_CHECK_NEAR(a.max, b.max, 1e-9);
    out = a;
  };

  figure.layout();
  plot::Range previous{};
  both_rows_hold(previous);
  // The window ends at the edge the range said it should end at, and is ten wide:
  // both members hold it, and the slower row's own edge is not where it ends.
  PLOT_CHECK_NEAR(previous.max, edge0, 1e-9);
  PLOT_CHECK_NEAR(previous.max - previous.min, 10.0, 1e-9);
  PLOT_CHECK(right.x_bounds().max < previous.max - 50.0);

  // The producer keeps going -- the two rows at different rates, so their *own* live
  // edges are different and a window derived from one row's stream would be
  // unmistakable -- and every frame moves *both* rows to the same window, the one the
  // group derives from the newest sample it can see.
  for (int step = 1; step <= 60; ++step) {
    for (int i = 0; i < 4; ++i) {
      left.push(static_cast<plot::f32>(std::sin(step * 0.1)));
    }
    for (int i = 0; i < 2; ++i) {
      right.push(static_cast<plot::f32>(std::cos(step * 0.1)));
    }
    left.acquire();
    right.acquire();
    figure.layout();
    plot::Range now{};
    both_rows_hold(now);
    PLOT_CHECK(now.max > previous.max);              // the window advanced
    PLOT_CHECK_NEAR(now.max - now.min, 10.0, 1e-9);  // and kept its span
    // The window's right end is the *newest* of the two rows' edges, which is the
    // faster row's -- the slower row's own edge would be well behind it.
    PLOT_CHECK(left.x_bounds().max > right.x_bounds().max + 10.0);
    PLOT_CHECK_NEAR(now.max, left.x_bounds().max, 1e-9);
    previous = now;
  }
  PLOT_CHECK(previous.max > edge0);

  // Panning a member while the group follows is a *span-and-offset* statement, so it
  // moves all rows together and leaves following on -- the group's edge still owns
  // the window's right end.
  figure.plot(1u).set_x_range(0u, {previous.max - 60.0, previous.max - 40.0});
  figure.layout();
  plot::Range panned{};
  both_rows_hold(panned);
  PLOT_CHECK_NEAR(panned.max - panned.min, 20.0, 1e-9);       // the span the pan stated
  PLOT_CHECK_NEAR(panned.max, left.x_bounds().max - 40.0, 1e-9);  // 40 behind the edge
  PLOT_CHECK(figure.x_link_follow(group) == plot::XFollow::Live);
  PLOT_CHECK(figure.plot(0u).x_follow(0u) == plot::XFollow::Off);  // members stay Off

  // Disarming leaves the window where it is; the rows are still one range.
  PLOT_CHECK(figure.set_x_link_follow(group, plot::XFollow::Off));
  const plot::Range frozen = figure.x_link_range(group);
  for (int frame = 0; frame < 5; ++frame) {
    left.push(1.0f);
    left.acquire();
    figure.layout();
    both_rows_hold(previous);
    PLOT_CHECK_NEAR(figure.x_link_range(group).min, frozen.min, 1e-9);
    PLOT_CHECK_NEAR(figure.x_link_range(group).max, frozen.max, 1e-9);
  }
}