diff --git a/piker/data/_formatters.py b/piker/data/_formatters.py
index 0c402aa4..9d8fc00d 100644
--- a/piker/data/_formatters.py
+++ b/piker/data/_formatters.py
@@ -278,9 +278,9 @@ class IncrementalFormatter(msgspec.Struct):
             post_slice,
         ) = self.diff(new_read)
 
+        # we first need to allocate xy data arrays
+        # from the source data.
         if self.y_nd is None:
-            # we first need to allocate xy data arrays
-            # from the source data.
             self.xy_nd_start = shm._first.value
             self.xy_nd_stop = shm._last.value
             self.x_nd, self.y_nd = self.allocate_xy_nd(
@@ -289,45 +289,52 @@ class IncrementalFormatter(msgspec.Struct):
             )
             profiler('allocated xy history')
 
-        if prepend_len:
-            self.incr_update_xy_nd(
-                shm,
-                array_key,
+        # once allocated we do incremental pre/append
+        # updates from the diff with the source buffer.
+        else:
+            if prepend_len:
 
-                # this is the pre-sliced, "normally expected"
-                # new data that an updater would normally be
-                # expected to process, however in some cases (like
-                # step curves) the updater routine may want to do
-                # the source history-data reading itself, so we pass
-                # both here.
-                shm._array[pre_slice],
-                pre_slice,
-                prepend_len,
+                self.incr_update_xy_nd(
+                    shm,
+                    array_key,
 
-                self.xy_nd_start,
-                self.xy_nd_stop,
-                is_append=False,
-            )
+                    # this is the pre-sliced, "normally expected"
+                    # new data that an updater would normally be
+                    # expected to process, however in some cases (like
+                    # step curves) the updater routine may want to do
+                    # the source history-data reading itself, so we pass
+                    # both here.
+                    shm._array[pre_slice],
+                    pre_slice,
+                    prepend_len,
 
-            # self.y_nd[y_nd_slc] = new_y_nd
-            self.xy_nd_start = shm._first.value
-            profiler('prepended xy history: {prepend_length}')
+                    self.xy_nd_start,
+                    self.xy_nd_stop,
+                    is_append=False,
+                )
 
-        if append_len:
-            self.incr_update_xy_nd(
-                shm,
-                array_key,
+                self.xy_nd_start -= prepend_len
+                profiler('prepended xy history: {prepend_length}')
 
-                shm._array[post_slice],
-                post_slice,
-                append_len,
+                xndall = self.x_nd[self.xy_slice]
+                if xndall.any() and (xndall == 0.5).any():
+                    breakpoint()
 
-                self.xy_nd_start,
-                self.xy_nd_stop,
-                is_append=True,
-            )
-            self.xy_nd_stop = shm._last.value
-            profiler('appened xy history: {append_length}')
+            if append_len:
+                self.incr_update_xy_nd(
+                    shm,
+                    array_key,
+
+                    shm._array[post_slice],
+                    post_slice,
+                    append_len,
+
+                    self.xy_nd_start,
+                    self.xy_nd_stop,
+                    is_append=True,
+                )
+                self.xy_nd_stop += append_len
+                profiler('appened xy history: {append_length}')
 
         view_changed: bool = False
         view_range: tuple[int, int] = (ivl, ivr)
@@ -491,9 +498,14 @@ class IncrementalFormatter(msgspec.Struct):
         '''
         # NOTE: we don't include the very last datum which is filled in
         # normally by another graphics object.
+        x_1d = array[self.index_field][:-1]
+        if x_1d.any() and (x_1d[-1] == 0.5).any():
+            breakpoint()
+
+        y_1d = array[array_key][:-1]
         return (
-            array[self.index_field][:-1],
-            array[array_key][:-1],
+            x_1d,
+            y_1d,
 
             # 1d connection array or style-key to
             # ``pg.functions.arrayToQPath()``
@@ -797,7 +809,7 @@ class StepCurveFmtr(IncrementalFormatter):
         if not x_1d.size == y_1d.size:
             breakpoint()
 
-        if x_1d.any() and (x_1d[-1] == 0.5).any():
+        if x_1d.any() and (x_1d == 0.5).any():
             breakpoint()
 
         # debugging