SignalXY: improve behavior of rotated and inverted plots

* Bugfix in end-point interpolation for rotated SignalXY

* Bugfix for lastpoint coordinates in rotated SignalXY plottables
(resolves issue discussed in #3812

* Bug fix for inverted Axes when finding last pixel for SignalXY plots

* Update CHANGELOG.md

---------

Co-authored-by: Scott W Harden <swharden@gmail.com>

CHANGELOG.md

@@ -32,14 +32,15 @@ _Not yet on NuGet..._
 * Scatter: Added `GetNearestX()` to the data source to help locate the point closest to the cursor's X position (#3807) @MatKinPro
 * Controls: Disable middle-click-drag zooming on axes which have no data (#3810, #3897) @MCF
 * DataLogger: Create `Add()` overloads which accept fixed-length arrays (#3555) @h25019871990
-* SignalXY: Ensure the final point is always drawn in high density mode (#3812)
+* SignalXY: Ensure the final point is always drawn in high density mode (#3812, #3812)
 * Axes: Improved exception messages when calling `Zoom()` methods with invalid scale factors (#3813) @KennyTK
 * WinForms: Exposed `SKControl` so users may bind to its events (#3819) @CD-SailingPerf
 * Scatter: Added support for `Scale` and `Offset` properties (#3835) @bukkideme
 * Axis Lines: Separated `LegendText` from `LabelText` so items may be configured separately
 * Heatmap: Exposed `CellWidth` and `CellHeight` as an alternative sizing strategy to setting `Extent` (#3869) @alexisvrignaud
 * ImageRect: New plot type that places an image inside a defined rectangle on the plot (#3870) @sdpenner
 * Axis Rules: Improved behavior of snapping rules and improve smoothness of panning rules (#3919, #3547, #3701) @BrianAtZetica
+* SignalXY: Improved appearance of rotated plots when low density mode is in use (#3921) @BrianAtZetica
 
 ## ScottPlot 5.0.34
 _Published on [NuGet](https://www.nuget.org/profiles/ScottPlot) on 2024-05-05_

src/ScottPlot5/ScottPlot5/DataSources/SignalXYSourceDoubleArray.cs

@@ -57,17 +57,18 @@ private Pixel[] GetPixelsToDrawHorizontally(RenderPack rp, IAxes axes)
             .Select(pxColumn => GetColumnPixelsX(pxColumn, visibileRange, rp, axes))
             .SelectMany(x => x);
 
+        // duplicate the last point to ensure it is always rendered
+        // https://github.com/ScottPlot/ScottPlot/issues/3812
+        Pixel lastPoint = axes.GetPixel(new Coordinates(Xs[dataIndexLast], Ys[dataIndexLast]));
+
         Pixel[] leftOutsidePoint = PointBefore, rightOutsidePoint = PointAfter;
         if (axes.XAxis.Range.Span < 0)
         {
             leftOutsidePoint = PointAfter;
             rightOutsidePoint = PointBefore;
+            lastPoint = axes.GetPixel(new Coordinates(Xs[dataIndexFirst], Ys[dataIndexFirst]));
         }
 
-        // duplicate the last point to ensure it is always rendered
-        // https://github.com/ScottPlot/ScottPlot/issues/3812
-        Pixel lastPoint = axes.GetPixel(new Coordinates(Xs[dataIndexLast], Ys[dataIndexLast]));
-
         // combine with one extra point before and after
         Pixel[] points = [.. leftOutsidePoint, .. VisiblePoints, .. rightOutsidePoint, lastPoint];
 
@@ -93,16 +94,18 @@ private Pixel[] GetPixelsToDrawVertically(RenderPack rp, IAxes axes)
             .Select(pxRow => GetColumnPixelsY(pxRow, visibleRange, rp, axes))
             .SelectMany(x => x);
 
+        // duplicate the last point to ensure it is always rendered
+        // https://github.com/ScottPlot/ScottPlot/issues/3812
+        Pixel lastPoint = axes.GetPixel(new Coordinates(Ys[dataIndexLast], Xs[dataIndexLast]));
+
         Pixel[] bottomOutsidePoint = PointBefore, topOutsidePoint = PointAfter;
         if (axes.YAxis.Range.Span < 0)
         {
             bottomOutsidePoint = PointAfter;
             topOutsidePoint = PointBefore;
+            lastPoint = axes.GetPixel(new Coordinates(Ys[dataIndexFirst], Xs[dataIndexFirst]));
         }
 
-        // duplicate the last point to ensure it is always rendered
-        // https://github.com/ScottPlot/ScottPlot/issues/3812
-        Pixel lastPoint = axes.GetPixel(new Coordinates(Xs[dataIndexLast], Ys[dataIndexLast]));
 
         // combine with one extra point before and after
         Pixel[] points = [.. bottomOutsidePoint, .. VisiblePoints, .. topOutsidePoint, lastPoint];
@@ -182,7 +185,7 @@ private IEnumerable<Pixel> GetColumnPixelsX(int pixelColumnIndex, IndexRange rng
             yield break;
         }
 
-        yield return new Pixel(xPixel, axes.GetPixelY(Ys[startIndex] * YScale + YOffset)); // enter
+        yield return new Pixel(xPixel, axes.GetPixelY(Ys[Math.Min(startIndex, endIndex)] * YScale + YOffset)); // enter
 
         if (pointsInRange > 1)
         {
@@ -215,7 +218,7 @@ private IEnumerable<Pixel> GetColumnPixelsY(int rowColumnIndex, IndexRange rng,
             yield break;
         }
 
-        yield return new Pixel(axes.GetPixelX(Ys[startIndex] + XOffset), yPixel); // enter
+        yield return new Pixel(axes.GetPixelX(Ys[Math.Min(startIndex,endIndex)] + XOffset), yPixel); // enter
 
         if (pointsInRange > 1)
         {
@@ -258,8 +261,8 @@ private IEnumerable<Pixel> GetColumnPixelsY(int rowColumnIndex, IndexRange rng,
 
         if (firstPointPosition > MinimumIndex)
         {
-            float beforeX = axes.GetPixelX(Xs[firstPointIndex - 1] + XOffset);
-            float beforeY = axes.GetPixelY(Ys[firstPointIndex - 1] * YScale + YOffset);
+            float beforeY = axes.GetPixelY(Xs[firstPointIndex - 1] + XOffset);
+            float beforeX = axes.GetPixelX(Ys[firstPointIndex - 1] * YScale + YOffset);
             Pixel beforePoint = new(beforeX, beforeY);
             return ([beforePoint], firstPointIndex);
         }
@@ -300,8 +303,8 @@ private IEnumerable<Pixel> GetColumnPixelsY(int rowColumnIndex, IndexRange rng,
 
         if (lastPointPosition <= MaximumIndex)
         {
-            float afterX = axes.GetPixelX(Xs[lastPointIndex] + XOffset);
-            float afterY = axes.GetPixelY(Ys[lastPointIndex] * YScale + YOffset);
+            float afterY = axes.GetPixelY(Xs[lastPointIndex] + XOffset);
+            float afterX = axes.GetPixelX(Ys[lastPointIndex] * YScale + YOffset);
             Pixel afterPoint = new(afterX, afterY);
             return ([afterPoint], lastPointIndex);
         }