feat: add linear regression trendline to revenue charts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/Dashboard.tsx

@@ -106,14 +106,44 @@ function Dashboard({ data, showDataLabels, setShowDataLabels, includeVAT, setInc
       return d.toLocaleDateString('en-US', { month: 'short', day: 'numeric' });
     };
 
+    // Linear regression helper
+    const linearRegression = (values: number[]) => {
+      const n = values.length;
+      if (n < 2) return values;
+      let sumX = 0, sumY = 0, sumXY = 0, sumX2 = 0;
+      for (let i = 0; i < n; i++) {
+        sumX += i;
+        sumY += values[i];
+        sumXY += i * values[i];
+        sumX2 += i * i;
+      }
+      const slope = (n * sumXY - sumX * sumY) / (n * sumX2 - sumX * sumX);
+      const intercept = (sumY - slope * sumX) / n;
+      return values.map((_, i) => slope * i + intercept);
+    };
+
+    const trendlineDataset = (values: number[]) => ({
+      label: 'Trend',
+      data: linearRegression(values),
+      borderColor: chartColors.secondary,
+      borderWidth: 2,
+      borderDash: [6, 4],
+      tension: 0,
+      fill: false,
+      pointRadius: 0,
+      pointHoverRadius: 0,
+      datalabels: { display: false }
+    });
+
     if (trendGranularity === 'week') {
       const grouped = groupByWeek(filteredData, includeVAT);
       const weeks = Object.keys(grouped).filter(w => w).sort();
+      const revenueValues = weeks.map(w => grouped[w].revenue);
       return {
         labels: weeks.map(formatLabel),
         datasets: [{
           label: includeVAT ? 'Revenue (incl. VAT)' : 'Revenue (excl. VAT)',
-          data: weeks.map(w => grouped[w].revenue),
+          data: revenueValues,
           borderColor: chartColors.primary,
           backgroundColor: chartColors.primary + '10',
           borderWidth: 2,
@@ -121,7 +151,7 @@ function Dashboard({ data, showDataLabels, setShowDataLabels, includeVAT, setInc
           fill: true,
           pointRadius: 0,
           pointHoverRadius: 4
-        }]
+        }, trendlineDataset(revenueValues)]
       };
     } else {
       // Daily granularity
@@ -132,11 +162,12 @@ function Dashboard({ data, showDataLabels, setShowDataLabels, includeVAT, setInc
         dailyData[date] += Number(row[revenueField] || row.revenue_incl_tax || 0);
       });
       const days = Object.keys(dailyData).sort();
+      const revenueValues = days.map(d => dailyData[d]);
       return {
         labels: days.map(formatLabel),
         datasets: [{
           label: includeVAT ? 'Revenue (incl. VAT)' : 'Revenue (excl. VAT)',
-          data: days.map(d => dailyData[d]),
+          data: revenueValues,
           borderColor: chartColors.primary,
           backgroundColor: chartColors.primary + '10',
           borderWidth: 1.5,
@@ -144,7 +175,7 @@ function Dashboard({ data, showDataLabels, setShowDataLabels, includeVAT, setInc
           fill: true,
           pointRadius: 0,
           pointHoverRadius: 3
-        }]
+        }, trendlineDataset(revenueValues)]
       };
     }
   }, [filteredData, trendGranularity, includeVAT]);
@@ -465,7 +496,7 @@ function Dashboard({ data, showDataLabels, setShowDataLabels, includeVAT, setInc
               </div>
             }
           >
-            <Line data={trendData} options={{...baseOptions, scales: {...baseOptions.scales, x: {...baseOptions.scales.x, ticks: {...baseOptions.scales.x.ticks, maxTicksLimit: trendGranularity === 'week' ? 15 : 20}}}}} />
+            <Line data={trendData} options={{...baseOptions, plugins: {...baseOptions.plugins, legend: {display: true, position: 'top', align: 'end', labels: {boxWidth: 12, padding: 12, font: {size: 13}}}}, scales: {...baseOptions.scales, x: {...baseOptions.scales.x, ticks: {...baseOptions.scales.x.ticks, maxTicksLimit: trendGranularity === 'week' ? 15 : 20}}}}} />
           </ExportableChart>
         </div>
 
@@ -556,7 +587,7 @@ function Dashboard({ data, showDataLabels, setShowDataLabels, includeVAT, setInc
                     <button className={trendGranularity === 'week' ? 'active' : ''} onClick={() => setTrendGranularity('week')}>{t('time.weekly')}</button>
                   </div>
                   <div className="chart-container">
-                    <Line data={trendData} options={{...baseOptions, scales: {...baseOptions.scales, x: {...baseOptions.scales.x, ticks: {...baseOptions.scales.x.ticks, maxTicksLimit: 8}}}}} />
+                    <Line data={trendData} options={{...baseOptions, plugins: {...baseOptions.plugins, legend: {display: true, position: 'top', align: 'end', labels: {boxWidth: 10, padding: 8, font: {size: 12}}}}, scales: {...baseOptions.scales, x: {...baseOptions.scales.x, ticks: {...baseOptions.scales.x.ticks, maxTicksLimit: 8}}}}} />
                   </div>
                 </div>
               </div>