fix(figure): a following group keeps its whole window on the data

The offset of a following group was clamped to `retained` -- measured from the newest sample -- so
a pan (or a zoom-in anchored near an edge) could put the window's *right* edge exactly on the oldest
retained sample. That leaves the window's entire span to its left over data the scope never had: an
empty margin as wide as the view. The bound is about the whole window, so it is `retained - span` --
but only while the window *can* sit on the data. When the members retain less than a window's worth
(a ring still filling, or one just trimmed) nothing avoids a margin, and the older placement is the
honest one and stays: right edge on the newest sample, the shortfall on the left.

Test: a_following_group_keeps_its_whole_window_on_the_data -- asks to scroll ten spans past the
oldest sample and asserts the window is still a window, with its left edge on the data. The
existing `a_following_log_group_holds_decades` codified the old bound (`max == edge/10` after asking
for a decade of pan with far less than that retained); its log-space subject is unchanged and the
offset assertion is now the invariant above.

tests/test_figure.cpp

@@ -844,20 +844,29 @@ PLOT_TEST(a_following_log_group_holds_decades) {
 
   // A member panned while the group follows: the adopted scroll state is the
   // adopted range's span *in decades* -- two here (edge/1000 to edge/10 is a
-  // factor of 100) -- and the offset one decade behind the edge, both of which
-  // survive the next frames' advances.
+  // factor of 100) -- and the offset as far behind the edge as the retained data
+  // allows, both of which survive the next frames' advances.
+  //
+  // The pan *asks* for a full decade back, and the group may not take it: the offset is bounded by
+  // `retained - span`, so the window stops with its left edge on the oldest sample. Asserting
+  // `max == edge/10` here was asserting the *old* bound, which put a whole span of the window over
+  // data the members never held -- the empty-left-margin defect this bound exists for. The
+  // log-space assertions above are this test's subject and are unchanged.
   const plot::f64 edge = left.x_bounds().max;
   figure.plot(1u).set_x_range(0u, {edge / 1000.0, edge / 10.0});
   figure.layout();
   PLOT_CHECK_NEAR(decades_of(figure.plot(0u)), 2.0, 1e-9);
   PLOT_CHECK_NEAR(decades_of(figure.plot(1u)), 2.0, 1e-9);
-  PLOT_CHECK_NEAR(figure.plot(0u).x_range(0u).max, edge / 10.0, 1e-8);
+  const plot::f64 oldest = std::min(left.x_bounds().min, right.x_bounds().min);
+  PLOT_CHECK(figure.plot(0u).x_range(0u).min >= oldest - 1e-6);
+  PLOT_CHECK(figure.plot(0u).x_range(0u).max <= edge + 1e-6);
   for (int step = 0; step < 3; ++step) {
     left.push(0.5f);
     left.acquire();
     figure.layout();
     PLOT_CHECK_NEAR(decades_of(figure.plot(0u)), 2.0, 1e-9);
     PLOT_CHECK_NEAR(decades_of(figure.plot(1u)), 2.0, 1e-9);
+    PLOT_CHECK(figure.plot(0u).x_range(0u).min >= oldest - 1e-6);
   }
   PLOT_CHECK(figure.x_link_follow(group) == plot::XFollow::Live);
 }