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

server.js

@@ -113,7 +113,10 @@ function buildChartOption(params) {
         leftLabel = '左侧',
         rightLabel = '右侧',
         splitStyle = 'solid',
-        seriesColors: customColors = null
+        seriesColors: customColors = null,
+        dualYAxis = false,
+        rightAxisSeries = null,
+        rightAxisName = ''
     } = params;
 
     const parsedData = parseData(data);
@@ -148,6 +151,12 @@ function buildChartOption(params) {
             }
         };
 
+        // 双Y轴：rightAxisSeries 指定右轴系列名
+        if (dualYAxis) {
+            const rightNames = Array.isArray(rightAxisSeries) ? rightAxisSeries : (rightAxisSeries ? [rightAxisSeries] : []);
+            seriesItem.yAxisIndex = rightNames.includes(name) ? 1 : 0;
+        }
+
         if (stackMode) {
             seriesItem.stack = 'total';
         }
@@ -159,8 +168,7 @@ function buildChartOption(params) {
             seriesItem.areaStyle = theme === 'gradient' ? { opacity: 0.15 } : undefined;
         }
 
-        if (type === 'bar') {
-            seriesItem.barMaxWidth = 40;
+        if (type === 'bar') {seriesItem.barMaxWidth = 40;
             seriesItem.itemStyle.borderRadius = stackMode ? [0, 0, 0, 0] : [4, 4, 0, 0];
         }
 
@@ -269,7 +277,33 @@ function buildChartOption(params) {
             },
             axisTick: { show: false }
         },
-        yAxis: {
+        yAxis: dualYAxis ? [
+            {
+                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 },
@@ -366,6 +400,9 @@ app.get('/api/chart', (req, res) => {
             leftLabel: req.query.leftLabel || '左侧',
             rightLabel: req.query.rightLabel || '右侧',
             splitStyle: req.query.splitStyle || 'solid',
+            dualYAxis: req.query.dualYAxis === 'true',
+            rightAxisSeries: req.query.rightAxisSeries ? req.query.rightAxisSeries.split(',') : null,
+            rightAxisName: req.query.rightAxisName || '',
             width: parseInt(req.query.width) || 800,
             height: parseInt(req.query.height) || 500,
             format: req.query.format || 'png',
@@ -882,7 +919,7 @@ app.get('/api/health', (req, res) => {
     res.json({
         status: 'ok',
         service: 'data-chart-tool',
-        version: '1.10.1',
+        version: '1.11.0',
         endpoints: {
             'POST /api/chart': '生成图表图片（JSON body）',
             'GET /api/chart': '生成图表图片（URL 参数）',
@@ -897,7 +934,7 @@ app.get('/api/health', (req, res) => {
 app.get('/api/docs', (req, res) => {
     res.json({
         name: '数据可视化图表生成器 API',
-        version: '1.10.1',
+        version: '1.11.0',
         endpoints: [
             {
                 method: 'POST',
@@ -922,7 +959,10 @@ app.get('/api/docs', (req, res) => {
                     width: { type: 'number', default: 800, description: '图片宽度(px)' },
                     height: { type: 'number', default: 500, description: '图片高度(px)' },
                     format: { type: 'string', default: 'png', options: ['png'], description: '输出格式' },
-                    pixelRatio: { type: 'number', default: 2, description: '像素倍率（清晰度）' }
+                    pixelRatio: { type: 'number', default: 2, description: '像素倍率（清晰度）' },
+                    dualYAxis: { type: 'boolean', default: false, description: '是否启用双Y轴（左右量度不同）' },
+                    rightAxisSeries: { type: 'array', default: null, description: '右轴系列名列表（如 ["利润"]，指定哪些系列用右轴）' },
+                    rightAxisName: { type: 'string', default: '', description: '右轴名称' }
                 },
                 returns: 'image/png',
                 example: {