fix: backend ORDER BY CASE for series sort, default value_asc

web/app/api/charts/route.ts

@@ -16,7 +16,7 @@ export const POST = withAuth(async (req: NextRequest) => {
     filter_combinators = null, filter_items = null,
     filter_items_exclude = null, filter_items_regex = false,
     y_min = null, y_max = null, y_scale = 'linear',
-    series_limit = 20, order_by = 'time',
+    series_limit = 20, order_by = 'value_asc',
   } = body;
 
   if (!title) return NextResponse.json({ error: 'title required' }, { status: 400 });