Minor refactoring of Formatter logic

Source/Charts/Formatters/DefaultAxisValueFormatter.swift

@@ -22,7 +22,7 @@ open class DefaultAxisValueFormatter: NSObject, IAxisValueFormatter
 
     @objc open var hasAutoDecimals: Bool = false
 
-    fileprivate var _formatter: NumberFormatter?
+    private var _formatter: NumberFormatter?
     @objc open var formatter: NumberFormatter?
     {
         get { return _formatter }
@@ -32,8 +32,9 @@ open class DefaultAxisValueFormatter: NSObject, IAxisValueFormatter
             _formatter = newValue
         }
     }
-
-    fileprivate var _decimals: Int?
+
+    // TODO: Documentation. Especially the nil case
+    private var _decimals: Int?
     open var decimals: Int?
     {
         get { return _decimals }
@@ -90,14 +91,7 @@ open class DefaultAxisValueFormatter: NSObject, IAxisValueFormatter
     open func stringForValue(_ value: Double,
                                axis: AxisBase?) -> String
     {
-        if block != nil
-        {
-            return block!(value, axis)
-        }
-        else
-        {
-            return formatter?.string(from: NSNumber(floatLiteral: value)) ?? ""
-        }
+        return block?(value, axis) ??
+            (formatter?.string(from: NSNumber(floatLiteral: value)) ?? "")
     }
-
 }

Source/Charts/Formatters/DefaultFillFormatter.swift

@@ -26,10 +26,7 @@ open class DefaultFillFormatter: NSObject, IFillFormatter
 
     @objc open var block: Block?
 
-    public override init()
-    {
-
-    }
+    public override init() { }
 
     @objc public init(block: @escaping Block)
     {
@@ -45,47 +42,21 @@ open class DefaultFillFormatter: NSObject, IFillFormatter
         dataSet: ILineChartDataSet,
         dataProvider: LineChartDataProvider) -> CGFloat
     {
-        if block != nil
+        guard block == nil else { return block!(dataSet, dataProvider) }
+        var fillMin = CGFloat(0.0)
+
+        if dataSet.yMax > 0.0 && dataSet.yMin < 0.0
         {
-            return block!(dataSet, dataProvider)
+            fillMin = 0.0
         }
-        else
+        else if let data = dataProvider.data
         {
-            var fillMin = CGFloat(0.0)
-
-            if dataSet.yMax > 0.0 && dataSet.yMin < 0.0
-            {
-                fillMin = 0.0
-            }
-            else
-            {
-                if let data = dataProvider.data
-                {
-                    var max: Double, min: Double
-
-                    if data.yMax > 0.0
-                    {
-                        max = 0.0
-                    }
-                    else
-                    {
-                        max = dataProvider.chartYMax
-                    }
-
-                    if data.yMin < 0.0
-                    {
-                        min = 0.0
-                    }
-                    else
-                    {
-                        min = dataProvider.chartYMin
-                    }
-
-                    fillMin = CGFloat(dataSet.yMin >= 0.0 ? min : max)
-                }
-            }
-
-            return fillMin
+            let max = data.yMax > 0.0 ? 0.0 : dataProvider.chartYMax
+            let min = data.yMin < 0.0 ? 0.0 : dataProvider.chartYMin
+
+            fillMin = CGFloat(dataSet.yMin >= 0.0 ? min : max)
         }
+
+        return fillMin
     }
 }

Source/Charts/Formatters/DefaultValueFormatter.swift

@@ -94,14 +94,7 @@ open class DefaultValueFormatter: NSObject, IValueFormatter
                              dataSetIndex: Int,
                              viewPortHandler: ViewPortHandler?) -> String
     {
-        if block != nil
-        {
-            return block!(value, entry, dataSetIndex, viewPortHandler)
-        }
-        else
-        {
-            return formatter?.string(from: NSNumber(floatLiteral: value)) ?? ""
-        }
+        return block?(value, entry, dataSetIndex, viewPortHandler) ??
+            (formatter?.string(from: NSNumber(floatLiteral: value)) ?? "")
     }
-
 }

Source/Charts/Formatters/IndexAxisValueFormatter.swift

@@ -53,12 +53,7 @@ open class IndexAxisValueFormatter: NSObject, IAxisValueFormatter
                              axis: AxisBase?) -> String
     {
         let index = Int(value.rounded())
-
-        if index < 0 || index >= _valueCount || index != Int(value)
-        {
-            return ""
-        }
-
+        guard values.indices.contains(index), index != Int(value) else { return "" }
         return _values[index]
     }
 }