feat: 新增双图合并模式，支持横排/竖排将两张图合并为一张图

- 后端新增 POST /api/combine 接口（chart1/chart2 + direction/gap/pixelRatio/background）
- 前端新增 🖼️ 双图合并模式：两张图独立配置数据/标题/类型/主题
- 横排（左右等高三张图）或竖排（上下等宽），一键切换实时预览
- 修复 @napi-rs/canvas echarts 渲染需先 toBuffer 再 dispose 的关键问题
- 更新 README/API 文档

server.js

@@ -1,6 +1,6 @@
 const express = require('express');
 const cors = require('cors');
-const { createCanvas, registerFont } = require('@napi-rs/canvas');
+const { createCanvas, registerFont, loadImage } = require('@napi-rs/canvas');
 const echarts = require('echarts');
 const path = require('path');
 
@@ -1000,17 +1000,126 @@ app.get('/api/docs', (req, res) => {
                 example: {
                     request: `curl "http://localhost:16016/api/table?data=产品,价格,库存\\n手机,2999,100\\n平板,1999,50&title=产品列表" -o table.png`
                 }
+            },
+            {
+                method: 'POST',
+                path: '/api/combine',
+                description: '将两张图表合并到一张图片中（支持横排/竖排）',
+                'Content-Type': 'application/json',
+                params: {
+                    chart1: { type: 'object', required: true, description: '第一张图配置（同 /api/chart 参数）' },
+                    chart2: { type: 'object', required: true, description: '第二张图配置（同 /api/chart 参数）' },
+                    direction: { type: 'string', default: 'horizontal', options: ['horizontal', 'vertical'], description: '排列方向：horizontal 横排（左右）/ vertical 竖排（上下）' },
+                    gap: { type: 'number', default: 24, description: '两图间距(px)' },
+                    pixelRatio: { type: 'number', default: 2, description: '像素倍率（清晰度）' },
+                    background: { type: 'string', default: '#ffffff', description: '背景色' }
+                },
+                returns: 'image/png',
+                example: {
+                    request: `curl -X POST http://localhost:16016/api/combine \\
+  -H "Content-Type: application/json" \\
+  -d '{
+    "chart1": {"data": "产品, Q1, Q2\\n手机, 1200, 1800\\n平板, 800, 950", "title": "2024年销售", "chartType": "bar"},
+    "chart2": {"data": "月份, 营收\\n1月, 500\\n2月, 680\\n3月, 820", "title": "营收趋势", "chartType": "line"},
+    "direction": "horizontal"
+  }' -o combine.png`,
+                    response: 'PNG 图片二进制流'
+                }
             }
         ]
     });
 });
 
+// ===== API: 双图合并生成图片 =====
+// 将两张图（图表/表格均可）合并到一张图片中，支持横排（左右）或竖排（上下）
+app.post('/api/combine', async (req, res) => {
+    try {
+        const body = req.body || {};
+        const chart1 = body.chart1 || {};
+        const chart2 = body.chart2 || {};
+        const direction = body.direction === 'vertical' ? 'vertical' : 'horizontal';
+        const gap = parseInt(body.gap) || 24;
+        const pixelRatio = parseInt(body.pixelRatio) || 2;
+        const background = body.background || '#ffffff';
+
+        if (!chart1.data || !chart2.data) {
+            return res.status(400).json({ error: 'chart1 和 chart2 都需要提供 data 参数（CSV 格式）' });
+        }
+
+        // 渲染单个子图（直接按目标尺寸渲染，避免二次缩放损失）
+        // 注意：必须先 toBuffer 再 chart.dispose()，dispose 会清空 canvas 内容！
+        const renderSub = async (params, w, h) => {
+            const option = buildChartOption(params);
+            const c = createCanvas(w * pixelRatio, h * pixelRatio);
+            const chart = echarts.init(c, null, {
+                renderer: 'canvas',
+                width: w,
+                height: h,
+                devicePixelRatio: pixelRatio
+            });
+            chart.setOption(option);
+            const buf = c.toBuffer('image/png');
+            chart.dispose();
+            const img = await loadImage(buf);
+            return { img, w, h };
+        };
+
+        const a = await renderSub(chart1, parseInt(chart1.width) || 640, parseInt(chart1.height) || 420);
+        const b = await renderSub(chart2, parseInt(chart2.width) || 640, parseInt(chart2.height) || 420);
+
+        let W, H, aRect, bRect;
+        if (direction === 'vertical') {
+            // 竖排（上下）：等宽对齐
+            const tw = Math.max(a.w, b.w);
+            aRect = { w: tw, h: Math.round(a.h * (tw / a.w)) };
+            bRect = { w: tw, h: Math.round(b.h * (tw / b.w)) };
+            W = tw;
+            H = aRect.h + bRect.h + gap;
+        } else {
+            // 横排（左右）：等高对齐
+            const th = Math.max(a.h, b.h);
+            aRect = { w: Math.round(a.w * (th / a.h)), h: th };
+            bRect = { w: Math.round(b.w * (th / b.h)), h: th };
+            W = aRect.w + bRect.w + gap;
+            H = th;
+        }
+
+        const canvas = createCanvas(W * pixelRatio, H * pixelRatio);
+        const ctx = canvas.getContext('2d');
+        ctx.fillStyle = background;
+        ctx.fillRect(0, 0, W * pixelRatio, H * pixelRatio);
+
+        if (direction === 'vertical') {
+            // 5 参数形式：drawImage(img, dx, dy, dw, dh)，源整图缩放到目标矩形
+            ctx.drawImage(a.img, 0, 0, aRect.w * pixelRatio, aRect.h * pixelRatio);
+            ctx.drawImage(b.img, 0, (aRect.h + gap) * pixelRatio, bRect.w * pixelRatio, bRect.h * pixelRatio);
+        } else {
+            ctx.drawImage(a.img, 0, 0, aRect.w * pixelRatio, aRect.h * pixelRatio);
+            ctx.drawImage(b.img, (aRect.w + gap) * pixelRatio, 0, bRect.w * pixelRatio, bRect.h * pixelRatio);
+        }
+
+        const buffer = canvas.toBuffer('image/png');
+        res.set({
+            'Content-Type': 'image/png',
+            'Content-Length': buffer.length,
+            'X-Combine-Direction': direction,
+            'X-Chart-Width': W,
+            'X-Chart-Height': H
+        });
+        res.send(buffer);
+    } catch (err) {
+        console.error('Combine error:', err);
+        res.status(500).json({ error: '双图合并失败: ' + err.message });
+    }
+});
+
 // ===== 启动服务 =====
 app.listen(PORT, '0.0.0.0', () => {
     console.log(`🚀 数据可视化图表生成器已启动`);
     console.log(`📊 Web UI:  http://0.0.0.0:${PORT}`);
     console.log(`📡 图表API: http://0.0.0.0:${PORT}/api/chart`);
     console.log(`📋 表格API: http://0.0.0.0:${PORT}/api/table`);
+    console.log(`🖼️  合并API: http://0.0.0.0:${PORT}/api/combine`);
     console.log(`📖 文档:   http://0.0.0.0:${PORT}/api/docs`);
     console.log(`❤️  健康:   http://0.0.0.0:${PORT}/api/health`);
 });