Martino Ferrari
2dcd1f1864
A host zooming into a sub-second window cannot label it absolutely: Time formatting uses three significant digits, so 12.345671s and 12.345672s are the same string and a 1 ms step around a multi-second time is unlabelled. The axis gains an `origin`: every value it writes is `v - origin`, and the ticks are generated on the shifted grid, so the window reads 0, 500us, 1ms. `origin_follows_range` re-takes the origin from the axis' own range on every refresh -- the mode a *moving* window needs, since the edge is derived from the newest sample at layout time and a value the host wrote a frame earlier is a frame of scroll stale. `format_time` now scales its sub-second values by prefix, the way `format_si` does on a y axis: the unit that keeps the mantissa in [1, 1000). A fixed millisecond unit printed 0.005ms where 5us is the reading, and 1e-05ms where 10ns is. An exact zero prints `0`, the same convention `format_si` states. The origin subtraction's dust is snapped by a *relative* epsilon, so a tick on the origin reads 0 rather than 1.78e-12ms. Tested by `ticks_are_written_relative_to_an_axis_origin`, `a_range_origin_puts_the_zero_tick_on_the_left_edge` and the extended `format_time_shapes`; the suite is 359 cases, 0 failed.
45 lines
1.8 KiB
C++
45 lines
1.8 KiB
C++
// SPDX-License-Identifier: MIT
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// SPDX-FileCopyrightText: 2026 Martino Ferrari
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#include <plot/plot/axis.hpp>
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#include <cmath>
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namespace plot {
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void Axis::format_value(f64 v, char* buf, usize n) const {
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// The one place a value on this axis becomes text, so the origin is applied here and
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// everything that goes through it -- tick labels, and the crosshair's readout -- reads in
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// the same frame of reference as the axis itself.
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// The subtraction leaves a residue when the value and the origin are the same distant number
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// (a tick exactly on the origin then formatted as `1.78e-12ms` instead of `0ms`). Snapping it
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// is a *relative* test, so a genuinely small value on a large axis is not swallowed -- what
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// this removes is arithmetic dust, not data.
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f64 shown = v - origin;
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if (std::fabs(shown) < 1.0e-12 * std::max(1.0, std::fabs(v))) shown = 0.0;
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if (format == TickFormat::Time) {
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format_time(shown, buf, n);
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return;
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}
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format_si(shown, buf, n);
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}
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void Axis::refresh(f32 px, f32 min_px, bool include_minors) {
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// The range first, so a following origin is this frame's edge and not the last one's: the
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// zero tick then sits exactly on the window start, which is the whole point of asking.
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if (origin_follows_range) origin = range.min;
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generate_ticks(range, px, min_px, ticks, include_minors, origin);
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if (format != TickFormat::Time) return;
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// generate_ticks labels with format_si; a time axis relabels its majors here,
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// because the tick generator has no business knowing about a clock -- through the one
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// function that says how a value on this axis is written.
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char buf[64];
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for (Tick& tick : ticks) {
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if (!tick.major) continue;
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format_value(tick.value, buf, sizeof buf);
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tick.label = buf;
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}
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}
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} // namespace plot
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