Only draw mx/mn line for last uppx's worth of datums

When using m4, we downsample to the max and min of each
pixel-column's-worth of data thus preserving range / dispersion details
whilst not drawing more graphics then can be displayed by the available
amount of horizontal pixels.

Take and apply this exact same concept to the "last datum" graphics
elements for any `Flow` that is reported as being in a downsampled
state:

- take the xy output from the `Curve.draw_last_datum()`,
- slice out all data that fits in the last pixel's worth of x-range
  by using the uppx,
- compute the highest and lowest value from that data,
- draw a singe line segment which spans this yrange thus creating
  a simple vertical set of pixels which are "filled in" and show the
  entire y-range for the most recent data "contained with that pixel".

piker/ui/_display.py

@@ -654,6 +654,19 @@ def graphics_update_cycle(
 
         # run synchronous update on all linked flows
         for curve_name, flow in chart._flows.items():
+
+            if (
+                not (do_rt_update or do_append)
+                and liv
+                # even if we're downsampled bigly
+                # draw the last datum in the final
+                # px column to give the user the mx/mn
+                # range of that set.
+            ):
+                # always update the last datum-element
+                # graphic for all flows
+                flow.draw_last(array_key=curve_name)
+
             # TODO: should the "main" (aka source) flow be special?
             if curve_name == chart.data_key:
                 continue

piker/ui/_flows.py

@@ -34,6 +34,7 @@ import numpy as np
 from numpy.lib import recfunctions as rfn
 import pyqtgraph as pg
 from PyQt5.QtGui import QPainterPath
+from PyQt5.QtCore import QLineF
 
 from ..data._sharedmem import (
     ShmArray,
@@ -335,12 +336,6 @@ class Flow(msgspec.Struct):  # , frozen=True):
     graphics: Curve
     _shm: ShmArray
 
-    draw_last: Optional[
-        Callable[
-            [np.ndarray, str],
-            tuple[np.ndarray]
-        ]
-    ] = None
     is_ohlc: bool = False
     render: bool = True  # toggle for display loop
 
@@ -594,7 +589,6 @@ class Flow(msgspec.Struct):  # , frozen=True):
             # TODO: append logic inside ``.render()`` isn't
             # correct yet for step curves.. remove this to see it.
             should_redraw = True
-            # draw_last = True
             slice_to_head = -2
 
         # downsampling incremental state checking
@@ -690,8 +684,47 @@ class Flow(msgspec.Struct):  # , frozen=True):
         graphics.update()
         profiler('.update()')
 
+        # track downsampled state
+        self._in_ds = r._in_ds
+
         return graphics
 
+    def draw_last(
+        self,
+        array_key: Optional[str] = None,
+
+    ) -> None:
+
+        # shm read and slice to view
+        (
+            xfirst, xlast, src_array,
+            ivl, ivr, in_view,
+        ) = self.read()
+
+        g = self.graphics
+        array_key = array_key or self.name
+        x, y = g.draw_last_datum(
+            g.path,
+            src_array,
+            src_array,
+            False,  # never reset path
+            array_key,
+        )
+
+        if self._in_ds:
+            # we only care about the last pixel's
+            # worth of data since that's all the screen
+            # can represent on the last column where
+            # the most recent datum is being drawn.
+            uppx = self._last_uppx
+            y = y[-uppx:]
+            ymn, ymx = y.min(), y.max()
+            # print(f'drawing uppx={uppx} mxmn line: {ymn}, {ymx}')
+            g._last_line = QLineF(
+                x[-2], ymn,
+                x[-1], ymx,
+            )
+
 
 def by_index_and_key(
     renderer: Renderer,