Simplify `Flow.maxmin()` block logics

piker/ui/_flows.py

@@ -387,10 +387,11 @@ class Flow(msgspec.Struct):  # , frozen=True):
         lbar: int,
         rbar: int,
 
-    ) -> tuple[float, float]:
+    ) -> Optional[tuple[float, float]]:
         '''
         Compute the cached max and min y-range values for a given
-        x-range determined by ``lbar`` and ``rbar``.
+        x-range determined by ``lbar`` and ``rbar`` or ``None``
+        if no range can be determined (yet).
 
         '''
         rkey = (lbar, rbar)
@@ -400,9 +401,8 @@ class Flow(msgspec.Struct):  # , frozen=True):
 
         shm = self.shm
         if shm is None:
-            mxmn = None
+            return None
 
-        else:  # new block for profiling?..
         arr = shm.array
 
         # build relative indexes into shm array
@@ -415,7 +415,7 @@ class Flow(msgspec.Struct):  # , frozen=True):
         ]
 
         if not slice_view.size:
-                mxmn = None
+            return None
 
         elif self.yrange:
             mxmn = self.yrange
@@ -434,8 +434,8 @@ class Flow(msgspec.Struct):  # , frozen=True):
             mxmn = ylow, yhigh
             # print(f'{self.name} MANUAL maxmin: {mxmin}')
 
-            if mxmn is not None:
-                # cache new mxmn result
+        # cache result for input range
+        assert mxmn
         self._mxmns[rkey] = mxmn
 
         return mxmn