diff --git a/piker/data/_sampling.py b/piker/data/_sampling.py
index 466ef0e7..10dc43f6 100644
--- a/piker/data/_sampling.py
+++ b/piker/data/_sampling.py
@@ -142,11 +142,17 @@ async def broadcast(
     shm: Optional[ShmArray] = None,
 
 ) -> None:
-    # broadcast the buffer index step to any subscribers for
-    # a given sample period.
+    '''
+    Broadcast the given ``shm: ShmArray``'s buffer index step to any
+    subscribers for a given sample period.
+
+    The sent msg will include the first and last index which slice into
+    the buffer's non-empty data.
+
+    '''
     subs = sampler.subscribers.get(delay_s, ())
 
-    last = -1
+    first = last = -1
 
     if shm is None:
         periods = sampler.ohlcv_shms.keys()
@@ -156,11 +162,16 @@ async def broadcast(
         if periods:
             lowest = min(periods)
             shm = sampler.ohlcv_shms[lowest][0]
+            first = shm._first.value
             last = shm._last.value
 
     for stream in subs:
         try:
-            await stream.send({'index': last})
+            await stream.send({
+                'first': first,
+                'last': last,
+                'index': last,
+            })
         except (
             trio.BrokenResourceError,
             trio.ClosedResourceError
diff --git a/piker/data/feed.py b/piker/data/feed.py
index 605349e9..848fcc10 100644
--- a/piker/data/feed.py
+++ b/piker/data/feed.py
@@ -795,6 +795,15 @@ async def manage_history(
 
                     # manually trigger step update to update charts/fsps
                     # which need an incremental update.
+                    # NOTE: the way this works is super duper
+                    # un-intuitive right now:
+                    # - the broadcaster fires a msg to the fsp subsystem.
+                    # - fsp subsys then checks for a sample step diff and
+                    #   possibly recomputes prepended history.
+                    # - the fsp then sends back to the parent actor
+                    #   (usually a chart showing graphics for said fsp)
+                    #   which tells the chart to conduct a manual full
+                    #   graphics loop cycle.
                     for delay_s in sampler.subscribers:
                         await broadcast(delay_s)
 
diff --git a/piker/fsp/_engine.py b/piker/fsp/_engine.py
index 0776c7a2..cf45c40e 100644
--- a/piker/fsp/_engine.py
+++ b/piker/fsp/_engine.py
@@ -369,7 +369,12 @@ async def cascade(
                     # always trigger UI refresh after history update,
                     # see ``piker.ui._fsp.FspAdmin.open_chain()`` and
                     # ``piker.ui._display.trigger_update()``.
-                    await client_stream.send('update')
+                    await client_stream.send({
+                        'fsp_update': {
+                            'key': dst_shm_token,
+                            'first': dst._first.value,
+                            'last': dst._last.value,
+                    }})
                     return tracker, index
 
                 def is_synced(
diff --git a/piker/ui/_display.py b/piker/ui/_display.py
index aa7761db..d0654b10 100644
--- a/piker/ui/_display.py
+++ b/piker/ui/_display.py
@@ -309,6 +309,7 @@ def graphics_update_cycle(
     ds: DisplayState,
     wap_in_history: bool = False,
     trigger_all: bool = False,  # flag used by prepend history updates
+    prepend_update_index: Optional[int] = None,
 
 ) -> None:
     # TODO: eventually optimize this whole graphics stack with ``numba``
@@ -368,6 +369,17 @@ def graphics_update_cycle(
         profiler('maxmin call')
         liv = r >= i_step  # the last datum is in view
 
+        if (
+            prepend_update_index is not None
+            and lbar > prepend_update_index
+        ):
+            # on a history update (usually from the FSP subsys)
+            # if the segment of history that is being prepended
+            # isn't in view there is no reason to do a graphics
+            # update.
+            log.debug('Skipping prepend graphics cycle: frame not in view')
+            return
+
         # don't real-time "shift" the curve to the
         # left unless we get one of the following:
         if (
@@ -639,7 +651,7 @@ async def display_symbol_data(
     )
 
     # historical data fetch
-    brokermod = brokers.get_brokermod(provider)
+    # brokermod = brokers.get_brokermod(provider)
 
     # ohlc_status_done = sbar.open_status(
     #     'retreiving OHLC history.. ',
diff --git a/piker/ui/_fsp.py b/piker/ui/_fsp.py
index 5ed85d9b..3d90f014 100644
--- a/piker/ui/_fsp.py
+++ b/piker/ui/_fsp.py
@@ -435,12 +435,16 @@ class FspAdmin:
             # wait for graceful shutdown signal
             async with stream.subscribe() as stream:
                 async for msg in stream:
-                    if msg == 'update':
+                    info = msg.get('fsp_update')
+                    if info:
                         # if the chart isn't hidden try to update
                         # the data on screen.
                         if not self.linked.isHidden():
                             log.info(f'Re-syncing graphics for fsp: {ns_path}')
-                            self.linked.graphics_cycle(trigger_all=True)
+                            self.linked.graphics_cycle(
+                                trigger_all=True,
+                                prepend_update_index=info['first'],
+                            )
                     else:
                         log.info(f'recved unexpected fsp engine msg: {msg}')