Trends: explicit series so each favourite draws its own line

LineMark lacked a series: value, so Swift Charts merged every
station's points into one polyline — only the first station's
line was recognisable. Adding series: per station splits the
chart into N coloured lines (legend colours now match).

FuelBoard/TrendsView.swift

@@ -205,15 +205,23 @@ struct TrendsView: View {
     }
 
     private var chart: some View {
-        Chart(displaySeries) { history in
+        // NOTE: each LineMark MUST carry an explicit `series:` — without it
+        // Swift Charts merges every station's points into ONE polyline
+        // (points connect across stations, so only the first station's line
+        // is recognisable). The outer ForEach keeps one chart with N series;
+        // per-mark foregroundStyle then colours each series from the palette.
+        Chart {
+            ForEach(displaySeries) { history in
                 ForEach(history.points) { point in
                     LineMark(
                         x: .value("Date", point.date),
-                    y: .value("Price", point.pence)
+                        y: .value("Price", point.pence),
+                        series: .value("Station", history.name)
                     )
                     .foregroundStyle(seriesColor(index(of: history.stationID)))
                 }
             }
+        }
         .chartXAxis {
             AxisMarks(values: .stride(by: .day, count: xStride)) { _ in
                 AxisGridLine()