return last index for every rounding

Source/Charts/Data/Implementations/Standard/ChartDataSet.swift

@@ -217,7 +217,7 @@ open class ChartDataSet: ChartBaseDataSet
         rounding: ChartDataSetRounding) -> Int
     {
         var closest = partitioningIndex { $0.x >= xValue }
-        guard closest < endIndex else { return rounding == .closest ? (endIndex-1) : -1 }
+        guard closest < endIndex else { return index(before: endIndex) }
 
         var closestXValue = self[closest].x