feat: 单图模式新增双Y轴 + 下载按钮缩小

- 图表模式新增双Y轴：左右两个坐标轴，量度可不同
  - 显示选项加"双Y轴"开关，启用后显示右轴名称输入
  - 每个系列在系列配置中可选择左轴/右轴
  - 后端 /api/chart 支持 dualYAxis + rightAxisSeries + rightAxisName（POST/GET 均可）
- 导出设置折叠栏下载按钮改小，避免挤占

app.js

@@ -3,6 +3,7 @@ let chartInstance = null;
 let parsedData = null;
 let seriesColors = [];
 let seriesOrder = [];
+let seriesAxis = []; // 每个系列所属轴：0=左轴 1=右轴
 let currentMode = 'chart'; // 'chart' or 'table'
 let tableImageBlob = null;
 
@@ -131,6 +132,7 @@ function generateChart() {
     seriesOrder = parsedData.seriesNames.map((_, i) => i);
     const palette = colorPalettes[document.getElementById('themeStyle').value] || colorPalettes.default;
     seriesColors = parsedData.seriesNames.map((_, i) => palette[i % palette.length]);
+    seriesAxis = parsedData.seriesNames.map(() => 0);
 
     // 渲染系列配置
     renderSeriesConfig();
@@ -172,6 +174,8 @@ function updateChart() {
     const stackMode = document.getElementById('stackMode').checked;
     const smoothLine = document.getElementById('smoothLine').checked;
     const enableSplit = document.getElementById('enableSplit').checked;
+    const dualAxis = document.getElementById('dualAxis').checked;
+    const rightAxisName = document.getElementById('rightAxisName').value;
 
     // 显示/隐藏分割配置
     document.getElementById('splitConfig').style.display = enableSplit ? 'block' : 'none';
@@ -248,6 +252,11 @@ function updateChart() {
             };
         }
 
+        // 双Y轴：按系列指定左右轴
+        if (dualAxis) {
+            seriesItem.yAxisIndex = (seriesAxis[origIdx] === 1) ? 1 : 0;
+        }
+
         return seriesItem;
     });
 
@@ -379,7 +388,30 @@ function updateChart() {
             },
             axisTick: { show: false }
         },
-        yAxis: {
+        yAxis: dualAxis ? [
+            {
+                type: 'value',
+                name: '左轴',
+                nameTextStyle: { color: textColor, fontSize: 11, padding: [0, 0, 0, 4] },
+                axisLine: { show: false },
+                axisTick: { show: false },
+                axisLabel: { color: textColor, fontSize: 11 },
+                splitLine: {
+                    show: showGrid,
+                    lineStyle: { color: theme === 'dark' ? '#333' : '#f0f0f0', type: 'dashed' }
+                }
+            },
+            {
+                type: 'value',
+                name: rightAxisName || '右轴',
+                position: 'right',
+                nameTextStyle: { color: textColor, fontSize: 11, padding: [0, 4, 0, 0] },
+                axisLine: { show: false },
+                axisTick: { show: false },
+                axisLabel: { color: textColor, fontSize: 11 },
+                splitLine: { show: false }
+            }
+        ] : {
             type: 'value',
             axisLine: { show: false },
             axisTick: { show: false },
@@ -466,6 +498,13 @@ function renderSeriesConfig() {
     seriesOrder.forEach((origIdx, displayIdx) => {
         const name = parsedData.seriesNames[origIdx];
         const color = seriesColors[origIdx];
+        const dualAxis = document.getElementById('dualAxis').checked;
+        const axisSelect = dualAxis ? `
+            <select onchange="updateSeriesAxis(${origIdx}, this.value)" title="所属坐标轴">
+                <option value="0" ${seriesAxis[origIdx] === 0 ? 'selected' : ''}>左轴</option>
+                <option value="1" ${seriesAxis[origIdx] === 1 ? 'selected' : ''}>右轴</option>
+            </select>
+        ` : '';
 
         const item = document.createElement('div');
         item.className = 'series-item';
@@ -481,6 +520,7 @@ function renderSeriesConfig() {
                 <option value="bar">柱状</option>
                 <option value="line">折线</option>
             </select>
+            ${axisSelect}
         `;
 
         // 拖拽事件
@@ -566,6 +606,19 @@ function updateSeriesType(origIdx, type) {
     updateChart();
 }
 
+// ===== 双Y轴 =====
+function onDualAxisChange() {
+    const on = document.getElementById('dualAxis').checked;
+    document.getElementById('rightAxisGroup').style.display = on ? 'block' : 'none';
+    renderSeriesConfig(); // 显示/隐藏系列轴选择
+    updateChart();
+}
+
+function updateSeriesAxis(origIdx, axis) {
+    seriesAxis[origIdx] = parseInt(axis) || 0;
+    updateChart();
+}
+
 // ===== 工具函数 =====
 function adjustColor(hex, amount) {
     hex = hex.replace('#', '');