Fix pp line label update logic

We weren't updating the LHS size labels on creation and we now use the
lot size digits to do so. Change `PositionTracker.update()` to
`.update_from_pp_msg()`.

piker/ui/_position.py

@@ -622,20 +622,22 @@ class PositionTracker:
         self.pp_label.update()
         self.size_label.update()
 
-    def update(
+    def update_from_pp_msg(
         self,
         msg: BrokerdPosition,
         position: Optional[Position] = None,
 
     ) -> None:
-        '''Update graphics and data from average price and size.
+        '''Update graphics and data from average price and size passed in our
+        EMS ``BrokerdPosition`` msg.
 
         '''
         # XXX: better place to do this?
         symbol = self.chart.linked.symbol
+        lot_size_digits = symbol.lot_size_digits
         avg_price, size = (
             round(msg['avg_price'], ndigits=symbol.tick_size_digits),
-            round(msg['size'], ndigits=symbol.lot_size_digits),
+            round(msg['size'], ndigits=lot_size_digits),
         )
 
         # live pp updates
@@ -643,7 +645,11 @@ class PositionTracker:
         pp.avg_price = avg_price
         pp.size = size
 
-        self.update_line(avg_price, size)
+        self.update_line(
+            avg_price,
+            size,
+            lot_size_digits,
+        )
 
         # label updates
         self.size_label.fields['slots_used'] = round(
@@ -739,6 +745,7 @@ class PositionTracker:
         self,
         price: float,
         size: float,
+        size_digits: int,
 
     ) -> None:
         '''Update personal position level line.
@@ -747,7 +754,8 @@ class PositionTracker:
         # do line update
         line = self.line
 
-        if line is None and size:
+        if size:
+            if line is None:
 
                 # create and show a pp line
                 line = self.line = position_line(
@@ -757,22 +765,21 @@ class PositionTracker:
                     color=self._color,
                     marker=self._level_marker,
                 )
-            line.show()
 
-        elif line:
+            else:
 
-            if size != 0.0:
                 line.set_level(price)
                 self._level_marker.level = price
                 self._level_marker.update()
+
             # update LHS sizing label
             line.update_labels({
                 'size': size,
+                'size_digits': size_digits,
                 'fiat_size': round(price * size, ndigits=2)
             })
             line.show()
 
-            else:
-                # remove pp line from view
+        elif line:  # remove pp line from view if it exists on a net-zero pp
             line.delete()
             self.line = None