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

Source/Charts/Renderers/LineChartRenderer.swift

@@ -354,17 +354,20 @@ 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(lastCoordinate.x) { continue }
+
+            // If both points lie right of the viewport, break out early.
+            if !viewPortHandler.isInBoundsRight(firstCoordinate.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 +454,52 @@ 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  by using a linear function.
+
+        Linear function (calculus): `y = ax + b`
+
+        Note: this method will not check for collision with the right edge of the view port, as we assume lines run from left
+        to right (e.g. `startPoint < endPoint`).
+
+     - 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 start- and/or endpoint fall within the viewport, bail out early.
+        if isInBounds(point: startPoint) || isInBounds(point: endPoint) { return true }
+        // check if x in bound when it's a vertical line
+        if startPoint.x == endPoint.x { return isInBoundsX(startPoint.x) }
+
+        // Calculate the slope (`a`) of the line (e.g. `a = (y2 - y1) / (x2 - x1)`).
+        let a = (endPoint.y - startPoint.y) / (endPoint.x - startPoint.x)
+        // Calculate the y-correction (`b`) of the line (e.g. `b = y1 - (a * x1)`).
+        let b = startPoint.y - (a * startPoint.x)
+
+        // Check for colission with the left edge of the view port (e.g. `y = (a * minX) + b`).
+        // if a is 0, it's a horizontal line; checking b here is still valid, as b is `point.y` all the time
+        if isInBoundsY((a * contentRect.minX) + b) { return true }
+
+        // Skip unnecessary check for collision with the right edge of the view port
+        // (e.g. `y = (a * maxX) + b`), as such a line will either begin inside the view port,
+        // or intersect the left, top or bottom edges of the view port. Leaving this logic here for clarity's sake:
+        // if isInBoundsY((a * contentRect.maxX) + b) { return true }
+
+        // While slope `a` can theoretically never be `0`, we should protect against division by zero.
+        guard a != 0 else { return false }
+
+        // Check for collision with the bottom edge of the view port (e.g. `x = (maxY - b) / a`).
+        if isInBoundsX((contentRect.maxY - b) / a) { return true }
+
+        // Check for collision with the top edge of the view port (e.g. `x = (minY - b) / a`).
+        if isInBoundsX((contentRect.minY - b) / a) { return true }
+
+        // This line does not intersect the view port.
+        return false
+    }
+
     /// The current x-scale factor
     @objc open var scaleX: CGFloat
     {