Record flags in history and draw them as steps

A bool was excluded from the history as "not a measurement", so a true/false
port had no curve in the node panel and none on an edge — only the word. It is
recorded as 0/1 now and drawn as steps, since a bezier through two states
slopes through readings that never happened. The axis is pinned to 0..1, so a
flag that was never on sits at the floor rather than mid-box.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/api/routes/flows.py

@@ -837,7 +837,7 @@ def read_message_history(
     """The recent values of one message, for plotting.
 
     ``message`` may be given bare or qualified; a message that never carried a
-    number comes back with an empty series.
+    number or a flag comes back with an empty series.
     """
     _read_flow(controller, name)
     key = qualify(name, message)

backend/fluksio/api/routes/messages.py

@@ -102,7 +102,7 @@ async def publish_message(
 
 @router.get("/{name}/history", response_model=MessagePoints)
 def read_message_history(name: str, controller: FlowControllerDep) -> Any:
-    """The series behind a chart. Numbers only — nothing else plots."""
+    """The series behind a chart. Numbers and flags — nothing else plots."""
     if controller.pipeline is None:
         return MessagePoints(message=name, numeric=False, points=[])
     series = controller.state.history(name)

backend/fluksio/flow/state.py

@@ -24,16 +24,20 @@ import redis
 #: not JSON at all, and a datetime serialises rather than raising.
 _JSON_OPTS = orjson.OPT_NON_STR_KEYS
 
-# A sparkline only means something for numbers, so the history keeps the values
-# it can plot and nothing else. 120 points fill a panel-wide chart while leaving
-# Redis a cache rather than a time-series database.
+# A sparkline only means something for a value that can be placed on an axis,
+# so the history keeps those and nothing else. 120 points fill a panel-wide
+# chart while leaving Redis a cache rather than a time-series database.
 HISTORY_LIMIT = 120
 
 
 def as_number(value: Any) -> float | None:
-    """The plottable form of a value, or None if it is not a number."""
-    # bool is an int subclass; a flag is not a measurement.
-    if isinstance(value, bool) or not isinstance(value, (int, float)):
+    """The plottable form of a value, or None if it cannot be placed on an axis."""
+    # A flag is not a measurement, but when it was on is worth seeing, and 0/1
+    # is what a step curve is drawn from. bool is an int subclass, so it would
+    # fall through the isinstance below either way.
+    if isinstance(value, bool):
+        return 1.0 if value else 0.0
+    if not isinstance(value, (int, float)):
         return None
     return float(value)