fix(plot): a degenerate range cannot strand a live axis, and the demo's follow check fails the run

src/plot/plot.cpp

@@ -167,8 +167,7 @@ void Plot::set_x_range(u32 axis, Range r) {
   if (axis >= x_axes_.size()) return;
   Axis& a = x_axes_[axis];
   Range live{};
-  if (a.follow == XFollow::Live && live_x_bounds(axis, live) && r.span() > 0.0 &&
-      std::isfinite(r.min) && std::isfinite(r.max)) {
+  if (a.follow == XFollow::Live && live_x_bounds(axis, live)) {
     // Under Live the caller is stating a *span* and where the window sits relative
     // to the live edge, not a pair of absolute positions: the next layout()
     // recomputes the window from the series' newest x, so storing `r` verbatim
@@ -178,8 +177,22 @@ void Plot::set_x_range(u32 axis, Range r) {
     // drag's data delta -- and place_live_x_window() clamps it and stores the
     // window that follows from it, so the state is consistent before the next
     // layout rather than only after it.
-    a.range = r;
-    a.offset = live.max - r.max;
+    //
+    // This branch is taken unconditionally on a live axis, and what it refuses to
+    // store is what would strand it: `place_live_x_window()` needs a positive span,
+    // and every gesture keeps a non-positive one out of its arithmetic, so a
+    // degenerate range stored here would leave the axis following nothing, movable
+    // by nothing, while `x_follow()` still said Live. An inverted range is a
+    // caller's ordering slip, not a request to break the axis' `min <= max`
+    // invariant -- the rest of the library reads a range as a range -- so it is
+    // normalised; a span that is not a span is ignored outright, leaving the span
+    // and offset the axis holds. `auto_fit_x()` is how a window is framed either
+    // way.
+    const f64 lo = std::min(r.min, r.max);
+    const f64 hi = std::max(r.min, r.max);
+    if (!std::isfinite(lo) || !std::isfinite(hi) || !(hi > lo)) return;
+    a.range = Range{lo, hi};
+    a.offset = live.max - hi;
     place_live_x_window(axis);
     a.auto_scale = false;  // as ever: a range a caller chose is not an auto-fit
     return;