Note the issues with the shared fsp array index..

piker/fsp/__init__.py

@@ -44,6 +44,10 @@ async def increment_signals(
     feed: Feed,
     dst_shm: 'SharedArray',  # noqa
 ) -> None:
+    """Increment the underlying shared memory buffer on every "increment"
+    msg received from the underlying data feed.
+
+    """
     async for msg in await feed.index_stream():
         array = dst_shm.array
         last = array[-1:].copy()
@@ -64,6 +68,7 @@ async def cascade(
 ) -> AsyncIterator[dict]:
     """Chain streaming signal processors and deliver output to
     destination mem buf.
+
     """
     src = attach_shm_array(token=src_shm_token)
     dst = attach_shm_array(readonly=False, token=dst_shm_token)
@@ -87,10 +92,30 @@ async def cascade(
             feed.shm,
         )
 
+        # TODO: XXX:
+        # THERE'S A BIG BUG HERE WITH THE `index` field since we're
+        # prepending a copy of the first value a few times to make
+        # sub-curves align with the parent bar chart.
+        # 
+        # This likely needs to be fixed either by,
+        # - manually assigning the index and historical data
+        #   seperately to the shm array (i.e. not using .push())
+        # - developing some system on top of the shared mem array that
+        #   is `index` aware such that historical data can be indexed
+        #   relative to the true first datum? Not sure if this is sane
+        #   for derivatives.
+
         # Conduct a single iteration of fsp with historical bars input
         # and get historical output
-        history = await out_stream.__anext__()
+        history_output = await out_stream.__anext__()
 
+        # build a struct array which includes an 'index' field to push 
+        # as history
+        history = np.array(
+            np.arange(len(history_output)),
+            dtype=dst.array.dtype
+        )
+        history[fsp_func_name] = history_output
 
         # TODO: talk to ``pyqtgraph`` core about proper way to solve this:
         # XXX: hack to get curves aligned with bars graphics: prepend