Improve chart UX: dynamic price line color, smooth historical data prefetch

- Add custom price line (green/red based on candle direction)
- Disable default orange price line visibility
- Chart scrolls to recent data on timeframe switch
- Implement dynamic buffer for historical data prefetch (2x visible bars)
- Silently refills buffer when below 80% threshold

src/api/dashboard/static/js/ui/chart.js

@@ -91,6 +91,17 @@ export class TradingDashboard {
             borderDownColor: '#ff9800',
             wickUpColor: '#ff9800',
             wickDownColor: '#ff9800',
+            lastValueVisible: false,
+            priceLineVisible: false,
+        });
+        
+        this.currentPriceLine = this.candleSeries.createPriceLine({
+            price: 0,
+            color: '#26a69a',
+            lineWidth: 1,
+            lineStyle: LightweightCharts.LineStyle.Dotted,
+            axisLabelVisible: true,
+            title: '',
         });
         
         this.initPriceScaleControls();
@@ -240,7 +251,7 @@ export class TradingDashboard {
                 this.candleSeries.setData(mergedData);
                 
                 if (fitToContent) {
-                    this.chart.timeScale().fitContent();
+                    this.chart.timeScale().scrollToRealTime();
                 } else if (visibleRange) {
                     this.chart.timeScale().setVisibleLogicalRange(visibleRange);
                 }
@@ -308,73 +319,44 @@ export class TradingDashboard {
     
     onVisibleRangeChange() {
         if (!this.hasInitialLoad || this.isLoading) {
-            console.log('Skipping range change:', { hasInitialLoad: this.hasInitialLoad, isLoading: this.isLoading });
             return;
         }
         
         const visibleRange = this.chart.timeScale().getVisibleLogicalRange();
         if (!visibleRange) {
-            console.log('No visible range');
             return;
         }
         
         const data = this.candleSeries.data();
         if (!data || data.length === 0) {
-            console.log('No data available');
             return;
         }
         
-        const barsFromLeft = visibleRange.from;
+        const visibleBars = Math.ceil(visibleRange.to - visibleRange.from);
-        const totalBars = data.length;
+        const bufferSize = visibleBars * 2;
+        const refillThreshold = bufferSize * 0.8;
+        const barsFromLeft = Math.floor(visibleRange.from);
         
-        console.log('Visible range:', { from: visibleRange.from, to: visibleRange.to, barsFromLeft, totalBars });
+        if (barsFromLeft < refillThreshold) {
-        
+            console.log(`Buffer low (${barsFromLeft} < ${refillThreshold.toFixed(0)}), silently prefetching ${bufferSize} candles...`);
-        if (barsFromLeft < 50) {
-            console.log('Near left edge (within 50 bars), loading historical data...');
             const oldestCandle = data[0];
             if (oldestCandle) {
-                this.loadHistoricalData(oldestCandle.time);
+                this.loadHistoricalData(oldestCandle.time, bufferSize);
             }
         }
     }
     
-    async loadHistoricalData(beforeTime) {
+    async loadHistoricalData(beforeTime, limit = 1000) {
         if (this.isLoading) {
-            console.log('Already loading, skipping...');
             return;
         }
         this.isLoading = true;
-        console.log(`Loading historical data for ${this.currentInterval} before ${beforeTime}`);
-        
-        const currentData = this.candleSeries.data();
-        const visibleRange = this.chart.timeScale().getVisibleLogicalRange();
-        let leftmostTime = null;
-        
-        let savedPriceRange = null;
-        const isAutoScale = this.priceScaleState?.autoScale !== false;
-        if (!isAutoScale) {
-            try {
-                savedPriceRange = this.candleSeries.priceScale().getVisiblePriceRange();
-                console.log('Saving price range:', savedPriceRange);
-            } catch (e) {
-                console.log('Could not save price range');
-            }
-        }
-        
-        if (visibleRange && currentData.length > 0) {
-            const leftmostIndex = Math.floor(visibleRange.from);
-            if (leftmostIndex >= 0 && leftmostIndex < currentData.length) {
-                leftmostTime = currentData[leftmostIndex].time;
-            }
-        }
         
         try {
             const endTime = new Date((beforeTime - 1) * 1000);
             
-            console.log(`Loading historical data before ${new Date((beforeTime - 1) * 1000).toISOString()}`);
-            
             const response = await fetch(
-                `/api/v1/candles?symbol=BTC&interval=${this.currentInterval}&end=${endTime.toISOString()}&limit=500`
+                `/api/v1/candles?symbol=BTC&interval=${this.currentInterval}&end=${endTime.toISOString()}&limit=${limit}`
             );
             
             if (!response.ok) {
@@ -398,25 +380,7 @@ export class TradingDashboard {
                 
                 this.candleSeries.setData(mergedData);
                 
-                if (leftmostTime) {
+                console.log(`Prefetched ${chartData.length} candles, total: ${mergedData.length}`);
-                    const idx = mergedData.findIndex(c => c.time === leftmostTime);
-                    if (idx >= 0) {
-                        this.chart.timeScale().setVisibleLogicalRange({ from: idx, to: idx + 50 });
-                    }
-                }
-                
-                if (!isAutoScale && savedPriceRange) {
-                    setTimeout(() => {
-                        try {
-                            this.candleSeries.priceScale().setVisiblePriceRange(savedPriceRange);
-                            console.log('Restored price range:', savedPriceRange);
-                        } catch (e) {
-                            console.log('Could not restore price range');
-                        }
-                    }, 50);
-                }
-                
-                console.log(`Loaded ${chartData.length} historical candles`);
             } else {
                 console.log('No more historical data available');
             }
@@ -504,6 +468,14 @@ export class TradingDashboard {
     updateStats(candle) {
         const price = candle.close;
         const change = ((price - candle.open) / candle.open * 100);
+        const isUp = candle.close >= candle.open;
+        
+        if (this.currentPriceLine) {
+            this.currentPriceLine.applyOptions({
+                price: price,
+                color: isUp ? '#26a69a' : '#ef5350',
+            });
+        }
         
         document.getElementById('currentPrice').textContent = price.toFixed(2);
         document.getElementById('currentPrice').className = 'stat-value ' + (change >= 0 ? 'positive' : 'negative');