A toner rate needs days behind it

tests/test_plugins/test_supply_history.py

@@ -339,3 +339,26 @@ def test_a_real_near_empty_reading_before_a_swap_is_kept():
     points = normalise(series([40, 30, 5, 100, 95, 90]))
     assert 5.0 in [level for _, level in points]
     assert len(find_replacements(points)) == 1
+
+
+def test_minutes_of_readings_do_not_forecast_weeks():
+    """Supply items are often polled every few minutes.
+
+    Four readings a quarter of an hour apart, with a 2 point drop between the
+    ends, used to extrapolate to nearly 200 percent a day - so a cartridge
+    sitting at 82 percent was forecast to run out in a fortnight. A rate needs
+    time behind it, and without it the honest answer is no estimate.
+    """
+    minutes = 5
+    detail = analyse(series([84, 83, 83, 82], hours=minutes / 60))
+    assert detail['burnrateperday'] is None
+    assert detail['daysleft'] is None
+    assert detail['reason'] == 'not enough history yet'
+    assert band(detail['daysleft']) is None      # lands in "No estimate yet"
+
+
+def test_a_run_with_enough_days_still_estimates():
+    """The guard must not silence a genuinely slow, genuinely long run."""
+    detail = analyse(series([84, 83, 82, 81, 80, 79]))
+    assert detail['burnrateperday'] == 1.0
+    assert detail['daysleft'] == 79