Fix stacked area chart order issue (#25)

* Update StackedAreaChartStyle.swift

Author: beader

Sources/Charts/Chart/Styles/Area/StackedAreaChartStyle.swift

@@ -7,13 +7,14 @@ public struct StackedAreaChartStyle: ChartStyle {
 
     public func makeBody(configuration: Self.Configuration) -> some View {
         ZStack {
-            ForEach(Array(configuration.dataMatrix.flipDirection().stacked().reversed().enumerated()), id: \.self.offset) { enumeratedData in
-                self.colors.prefix(configuration.dataMatrix.count).reversed()[enumeratedData.offset % self.colors.count].clipShape(
+            ForEach(Array(configuration.dataMatrix.transpose().stacked().enumerated()), id: \.self.offset) { enumeratedData in
+                colors[enumeratedData.offset % colors.count].clipShape(
                     AreaChart(
                         unitData: enumeratedData.element,
                         lineType: self.lineType
                     )
                 )
+                .zIndex(-Double(enumeratedData.offset))
             }
         }
         .drawingGroup()
@@ -38,14 +39,14 @@ extension Collection where Element == [CGFloat] {
 }
 
 extension Array where Element == [CGFloat] {
-    func flipDirection() -> [[CGFloat]] {
+    func transpose() -> [[CGFloat]] {
         let columnsCount = self.first?.count ?? 0
         let rowCount = self.count
 
         return (0..<columnsCount).map { columnIndex in
             (0..<rowCount).map { rowIndex in
                 return self[rowIndex][columnIndex]
-            }.reversed()
+            }
         }
     }
 }