Fix TypeScript errors in toPolyline function

Accept undefined arrays and use non-null assertions for
array indexing after length check.

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

frontend/app/pages/commune/[slug]/index.vue

@@ -529,16 +529,17 @@ const gridPrices2 = computed(() => {
   return arr
 })
 
-function toPolyline(vols: number[], vals: number[], cyFn: (v: number) => number) {
-  if (!vols?.length) return ''
-  // Downsample for performance (every 4th point)
-  return vols
-    .filter((_: number, i: number) => i % 4 === 0 || i === vols.length - 1)
-    .map((_: number, i: number) => {
-      const idx = i * 4 >= vols.length ? vols.length - 1 : i * 4
-      return `${cx2(vols[idx])},${cyFn(vals[idx])}`
-    })
-    .join(' ')
+function toPolyline(vols: number[] | undefined, vals: number[] | undefined, cyFn: (v: number) => number): string {
+  if (!vols?.length || !vals?.length) return ''
+  const pts: string[] = []
+  for (let i = 0; i < vols.length; i += 4) {
+    pts.push(`${cx2(vols[i]!)},${cyFn(vals[i]!)}`)
+  }
+  const last = vols.length - 1
+  if (last % 4 !== 0) {
+    pts.push(`${cx2(vols[last]!)},${cyFn(vals[last]!)}`)
+  }
+  return pts.join(' ')
 }
 
 const baselineBillRP = computed(() => toPolyline(curveData.value?.baseline_volumes, curveData.value?.baseline_bills_rp, cy2bill))