fix: convert price data to float in performance chart

Prices from historical data may be stored as strings. Added explicit
float conversion before numeric operations to prevent TypeError when
comparing or calculating normalized values for both portfolio assets
and benchmark data.

backend/visualizations/plotly_charts.py

@@ -312,6 +312,9 @@ def create_performance_chart(
         if not dates or not prices:
             continue
 
+        # Convert prices to float in case they're strings
+        prices = [float(p) for p in prices]
+
         # Calculate percentage returns (normalise to 100)
         if prices[0] > 0:
             normalized = [(p / prices[0]) * 100 for p in prices]
@@ -337,22 +340,26 @@ def create_performance_chart(
         benchmark_dates = benchmark_data.get('dates', [])
         benchmark_prices = benchmark_data.get('close_prices', [])
 
-        if benchmark_dates and benchmark_prices and benchmark_prices[0] > 0:
-            normalized_benchmark = [(p / benchmark_prices[0]) * 100 for p in benchmark_prices]
-
-            fig.add_trace(go.Scatter(
-                x=benchmark_dates,
-                y=normalized_benchmark,
-                name="S&P 500 (Benchmark)",
-                mode='lines',
-                line=dict(width=3, dash='dash', color='#FFA500'),
-                hovertemplate=(
-                    "<b>S&P 500</b><br>" +
-                    "%{x|%B %d, %Y}<br>" +
-                    "Value: %{y:.2f}<br>" +
-                    "<extra></extra>"
-                )
-            ))
+        if benchmark_dates and benchmark_prices:
+            # Convert benchmark prices to float in case they're strings
+            benchmark_prices = [float(p) for p in benchmark_prices]
+
+            if benchmark_prices[0] > 0:
+                normalized_benchmark = [(p / benchmark_prices[0]) * 100 for p in benchmark_prices]
+
+                fig.add_trace(go.Scatter(
+                    x=benchmark_dates,
+                    y=normalized_benchmark,
+                    name="S&P 500 (Benchmark)",
+                    mode='lines',
+                    line=dict(width=3, dash='dash', color='#FFA500'),
+                    hovertemplate=(
+                        "<b>S&P 500</b><br>" +
+                        "%{x|%B %d, %Y}<br>" +
+                        "Value: %{y:.2f}<br>" +
+                        "<extra></extra>"
+                    )
+                ))
 
     theme = get_chart_theme()