LabelStyle: Add underline support (ScottPlot#4893)

* Finalize underline text logic for both LTR and RTL.

* Cookbook: underline recipe

* Underline: do not draw when width is 0

* Update CHANGELOG.md

---------

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

Author: manaruto

CHANGELOG.md

@@ -19,6 +19,7 @@ _Not yet on NuGet..._
 * GitHub: Create a `devconainer.json` file to facilitate development in GitHub Codespaces (#5002) @oxygen-dioxide
 * Maui: Improve behavior for interactive plots when their user input processor is disabled (#4990, #4989) @Adam--
 * Fonts: Added new styling options for weight, slant, density, etc. (#5013, #4873) @aespitia @Christoph-Wagner
+* Labels: Added styling options for underline with customizable thickness and offset (#4893) @manaruto
 
 ## ScottPlot 5.0.55
 _Published on [NuGet](https://www.nuget.org/profiles/ScottPlot) on 2025-03-22_

src/ScottPlot5/ScottPlot5 Cookbook/Recipes/General/Styling.cs

@@ -421,4 +421,28 @@ public override void Execute()
             myPlot.HideGrid();
         }
     }
+
+    public class SetFontUnderline : RecipeBase
+    {
+        public override string Name => "Set Label Underline";
+        public override string Description => "Underlines may be added to label styles. " +
+            "Underline thickness and offset may be customized as well.";
+
+        [Test]
+        public override void Execute()
+        {
+            for (int i = 0; i < 5; i++)
+            {
+                var text = myPlot.Add.Text($"Underline {i}px", i / 5.0, i);
+                text.LabelFontSize = 24;
+                text.LabelFontColor = ScottPlot.Palette.Default.GetColor(i);
+                text.LabelUnderline = true;
+                text.LabelUnderlineWidth = i;
+                text.LabelUnderlineOffset = 2 + i / 2;
+            }
+
+            myPlot.Axes.SetLimits(-1, 5, -1, 4);
+            myPlot.HideGrid();
+        }
+    }
 }

src/ScottPlot5/ScottPlot5/Primitives/LabelStyle.cs

@@ -48,6 +48,10 @@ public class LabelStyle
     public string FontName { get; set; } = Fonts.Default;
     public float FontSize { get; set; } = 12;
     public bool Bold { get; set; } = false;
+    public bool Underline { get; set; } = false;
+    private bool RenderUnderline => Underline & UnderlineWidth != 0;
+    public double UnderlineWidth { get; set; } = 1;
+    public double UnderlineOffset { get; set; } = 2;
 
     /// <summary>
     /// Manually defined line height in pixels.
@@ -329,10 +333,45 @@ private void DrawText(SKCanvas canvas, MeasuredText measured, SKPaint paint, Pix
                 if (LabelStyle.RTLSupport)
                 {
                     using (var shaper = new SKShaper(paint.Typeface))
+                    {
+                        float shapedWidth = shaper.Shape(lines[i], paint).Width;
                         canvas.DrawShapedText(shaper, lines[i], xPx, yPx, paint);
+
+                        if (RenderUnderline)
+                        {
+                            float underlineY = yPx + (float)UnderlineOffset;
+
+                            using var underlinePaint = new SKPaint
+                            {
+                                Color = paint.Color,
+                                StrokeWidth = (float)UnderlineWidth,
+                                IsStroke = true,
+                                IsAntialias = paint.IsAntialias
+                            };
+
+                            canvas.DrawLine(xPx, underlineY, xPx + shapedWidth, underlineY, underlinePaint);
+                        }
+                    }
                 }
                 else
+                {
                     canvas.DrawText(lines[i], xPx, yPx, paint);
+
+                    if (RenderUnderline)
+                    {
+                        float underlineY = yPx + (float)UnderlineOffset;
+                        float textWidth = paint.MeasureText(lines[i]);
+
+                        using var underlinePaint = new SKPaint
+                        {
+                            Color = paint.Color,
+                            StrokeWidth = (float)UnderlineWidth,
+                            IsStroke = true,
+                            IsAntialias = paint.IsAntialias
+                        };
+                        canvas.DrawLine(xPx, underlineY, xPx + textWidth, underlineY, underlinePaint);
+                    }
+                }
             }
         }
         else
@@ -342,10 +381,45 @@ private void DrawText(SKCanvas canvas, MeasuredText measured, SKPaint paint, Pix
             if (LabelStyle.RTLSupport)
             {
                 using (var shaper = new SKShaper(paint.Typeface))
+                {
+                    float shapedWidth = shaper.Shape(Text, paint).Width;
                     canvas.DrawShapedText(shaper, Text, xPx, yPx, paint);
+
+                    if (RenderUnderline)
+                    {
+                        float underlineY = yPx + (float)UnderlineOffset;
+
+                        using var underlinePaint = new SKPaint
+                        {
+                            Color = paint.Color,
+                            StrokeWidth = (float)UnderlineWidth,
+                            IsStroke = true,
+                            IsAntialias = paint.IsAntialias
+                        };
+
+                        canvas.DrawLine(xPx, underlineY, xPx + shapedWidth, underlineY, underlinePaint);
+                    }
+                }
             }
             else
+            {
                 canvas.DrawText(Text, xPx, yPx, paint);
+
+                if (RenderUnderline)
+                {
+                    float underlineY = yPx + (float)UnderlineOffset;
+                    float textWidth = paint.MeasureText(Text);
+
+                    using var underlinePaint = new SKPaint
+                    {
+                        Color = paint.Color,
+                        StrokeWidth = (float)UnderlineWidth,
+                        IsStroke = true,
+                        IsAntialias = paint.IsAntialias
+                    };
+                    canvas.DrawLine(xPx, underlineY, xPx + textWidth, underlineY, underlinePaint);
+                }
+            }
         }
     }
 

src/ScottPlot5/ScottPlot5/Primitives/LabelStyleProperties.cs

@@ -23,7 +23,9 @@ public abstract class LabelStyleProperties : IHasLabel
     public float? LabelLineSpacing { get => LabelStyle.LineSpacing; set => LabelStyle.LineSpacing = value; }
     public bool LabelItalic { get => LabelStyle.Italic; set => LabelStyle.Italic = value; }
     public bool LabelBold { get => LabelStyle.Bold; set => LabelStyle.Bold = value; }
-
+    public bool LabelUnderline { get => LabelStyle.Underline; set => LabelStyle.Underline = value; }
+    public double LabelUnderlineWidth { get => LabelStyle.UnderlineWidth; set => LabelStyle.UnderlineWidth = value; }
+    public double LabelUnderlineOffset { get => LabelStyle.UnderlineOffset; set => LabelStyle.UnderlineOffset = value; }
     public Color LabelFontColor { get => LabelStyle.ForeColor; set => LabelStyle.ForeColor = value; }
     public Color LabelBackgroundColor { get => LabelStyle.BackgroundColor; set => LabelStyle.BackgroundColor = value; }
     public float LabelBorderRadius { get => LabelStyle.BorderRadius; set => LabelStyle.BorderRadius = value; }