Fixes #4099: Line renderer did not render lines if their coordinates fell outside of the viewport.

Source/Charts/Renderers/LineChartRenderer.swift

@@ -353,17 +353,20 @@ open class LineChartRenderer: LineRadarRenderer
                 _lineSegments[i] = _lineSegments[i].applying(valueToPixelMatrix)
             }
 
-            if (!viewPortHandler.isInBoundsRight(_lineSegments[0].x))
-            {
-                break
-            }
+            // Determine the start and end coordinates of the line, and make sure they differ.
+            guard
+                let firstCoordinate = _lineSegments.first,
+                let lastCoordinate = _lineSegments.last,
+                firstCoordinate != lastCoordinate else { continue }
 
-            // make sure the lines don't do shitty things outside bounds
-            if !viewPortHandler.isInBoundsLeft(_lineSegments[1].x)
-                || (!viewPortHandler.isInBoundsTop(_lineSegments[0].y) && !viewPortHandler.isInBoundsBottom(_lineSegments[1].y))
-            {
-                continue
-            }
+            // If both points lie left of viewport, skip stroking.
+            if !viewPortHandler.isInBoundsLeft(firstCoordinate.x) && !viewPortHandler.isInBoundsLeft(lastCoordinate.x) { continue }
+
+            // If both points lie right of the viewport, break out early.
+            if !viewPortHandler.isInBoundsRight(firstCoordinate.x) && !viewPortHandler.isInBoundsRight(lastCoordinate.x) { break }
+
+            // Only stroke the line if it intersects with the viewport.
+            guard viewPortHandler.isIntersectingLine(from: firstCoordinate, to: lastCoordinate) else { continue }
 
             // get the color that is set for this line-segment
             context.setStrokeColor(dataSet.color(atIndex: j).cgColor)

Source/Charts/Utils/ViewPortHandler.swift

@@ -416,6 +416,17 @@ open class ViewPortHandler: NSObject
         return isInBoundsTop(y) && isInBoundsBottom(y)
     }
 
+    /**
+     A method to check whether coordinate lies within the viewport.
+
+     - Parameters:
+         - point: a coordinate.
+     */
+    @objc open func isInBounds(point: CGPoint) -> Bool
+    {
+        return isInBounds(x: point.x, y: point.y)
+    }
+
     @objc open func isInBounds(x: CGFloat, y: CGFloat) -> Bool
     {
         return isInBoundsX(x) && isInBoundsY(y)
@@ -443,6 +454,37 @@ open class ViewPortHandler: NSObject
         return (_contentRect.origin.y + _contentRect.size.height) >= normalizedY
     }
 
+    /**
+     A method to check whether a line between two coordinates intersects with the view port.
+
+     - Parameters:
+        - startPoint: the start coordinate of the line.
+        - endPoint: the end coordinate of the line.
+     */
+    @objc open func isIntersectingLine(from startPoint: CGPoint, to endPoint: CGPoint) -> Bool
+    {
+        // If the start or endpoint fall within the viewport, bail out early.
+        if isInBounds(point: startPoint) || isInBounds(point: endPoint) { return true }
+
+        // Calculate the slope of the line.
+        let slope = (endPoint.y - startPoint.y) / (endPoint.x - startPoint.x)
+
+        // Check for colission with the left edge of the view port.
+        if isInBoundsY((slope * (contentRect.minX - startPoint.x)) + startPoint.y) { return true }
+
+        // Check for colission with the right edge of the view port.
+        if isInBoundsY((slope * (contentRect.maxX - startPoint.x)) + startPoint.y) { return true }
+
+        // Check for colission with the top edge of the view port.
+        if isInBoundsX(((contentRect.minY - startPoint.y) / slope) + startPoint.x) { return true }
+
+        // Check for colission with the bottom edge of the viewport.
+        if isInBoundsX(((contentRect.maxY - startPoint.y) / slope) + startPoint.x) { return true }
+
+        // This line does not intersect the view port.
+        return false
+    }
+
     /// The current x-scale factor
     @objc open var scaleX: CGFloat
     {