Improve dashboard chart visibility and date handling

Author: HanSun103

docs/LIVE_FORWARD_MONITOR.md

@@ -107,6 +107,11 @@ previous official close, not to the session open.
 The date controls aggregate every completed observation in the selected range. Because the top-40
 holdings can change each day, each session uses the list frozen before that session.
 
+The Portfolio vs Benchmark chart uses the union of all evaluated dates. A model introduced later
+starts at its own first frozen observation and does not truncate the earlier history of established
+models. Click a model chip above the chart to hide or restore that series. Visibility choices and
+the selected date range persist across manual refreshes.
+
 The stock-level view defaults to the newest decision date with a completed holding-level evaluation.
 The newest frozen portfolios remain available in the selector with a `Pending` label until their next
 session closes. Experiment history defaults to the newest evaluation date and can be filtered by