diff --git a/piker/ui/_dataviz.py b/piker/ui/_dataviz.py
index 3bdfbe1e..55d2ff91 100644
--- a/piker/ui/_dataviz.py
+++ b/piker/ui/_dataviz.py
@@ -934,16 +934,17 @@ class Viz(msgspec.Struct):  # , frozen=True):
         # the most recent datum is being drawn.
         uppx = ceil(gfx.x_uppx())
 
+        alt_renderer = self._alt_r
+        if alt_renderer:
+            renderer, gfx = alt_renderer
+        else:
+            renderer = self._src_r
+
         if (
             (self._in_ds or only_last_uppx)
             and uppx > 0
+            and renderer is not None
         ):
-            alt_renderer = self._alt_r
-            if alt_renderer:
-                renderer, gfx = alt_renderer
-            else:
-                renderer = self._src_r
-
             fmtr = renderer.fmtr
             x = fmtr.x_1d
             y = fmtr.y_1d
@@ -952,23 +953,24 @@ class Viz(msgspec.Struct):  # , frozen=True):
             if alt_renderer:
                 iuppx = ceil(uppx / fmtr.flat_index_ratio)
 
-            y = y[-iuppx:]
-            ymn, ymx = y.min(), y.max()
-            try:
-                x_start = x[-iuppx]
-            except IndexError:
-                # we're less then an x-px wide so just grab the start
-                # datum index.
-                x_start = x[0]
+            if y is not None:
+                y = y[-iuppx:]
+                ymn, ymx = y.min(), y.max()
+                try:
+                    x_start = x[-iuppx]
+                except IndexError:
+                    # we're less then an x-px wide so just grab the start
+                    # datum index.
+                    x_start = x[0]
 
-            gfx._last_line = QLineF(
-                x_start, ymn,
-                x[-1], ymx,
-            )
-            # print(
-            #     f'updating DS curve {self.name}@{time_step}s\n'
-            #     f'drawing uppx={uppx} mxmn line: {ymn}, {ymx}'
-            # )
+                gfx._last_line = QLineF(
+                    x_start, ymn,
+                    x[-1], ymx,
+                )
+                # print(
+                #     f'updating DS curve {self.name}@{time_step}s\n'
+                #     f'drawing uppx={uppx} mxmn line: {ymn}, {ymx}'
+                # )
 
         else:
             x, y = gfx.draw_last_datum(