Add demo for logarithmic histogram plotting and fix rendering (oxyplot#740) (oxyplot#1886)

* Add demo for logarithmic histogram plotting and fix rendering (oxyplot#740)

* Fix UpdateMinMax for logarithmic scale in HistogramSeries.

* Omit invalid values in HistogramSeries when using logarithmic scale.

* Fix typo in UpdateMaxMinXY for HistogramSeries.

Author: FabianNitsche

CHANGELOG.md

@@ -25,6 +25,7 @@ All notable changes to this project will be documented in this file.
 - Code generation for escape sequences in strings (#1824)
 - Axes not always honoring AbsoluteMinimum/AbsoluteMaximum and/or MinimumRange/MaximumRange properties (#1812)
 - WindowsForms tracker no longer clipping outside PlotView boundaries (#1863)
+- Histogram now rendering properly when using logarithmic Y axis (#740) 
 
 ## [2.1.0] - 2021-10-02
 

CONTRIBUTORS

@@ -50,6 +50,7 @@ Elia Herzog
 Elmar Strittmatter
 episage <tilosag@gmail.com>
 eric
+Fabian Nitsche <ifab@freenet.de>
 Federico Coppola <fede@silentman.it>
 Francois Botha <igitur@gmail.com>
 Frank Tore Sæther <frank.sather@gmail.com>

Source/Examples/ExampleLibrary/Series/HistogramSeriesExamples.cs

@@ -29,6 +29,13 @@ public static PlotModel ExponentialDistribution()
             return CreateExponentialDistribution();
         }
 
+        [Example("Exponential Distribution (logarithmic)")]
+        [DocumentationExample("Series/HistogramSeries")]
+        public static PlotModel ExponentialDistributionLogarithmicAxis()
+        {
+            return CreateExponentialDistribution(true);
+        }
+
         [Example("Label Placement")]
         public static PlotModel HistogramLabelPlacement()
         {
@@ -122,10 +129,13 @@ public CustomHistogramItem(double rangeStart, double rangeEnd, double area, int
             public string Description { get; }
         }
 
-        public static PlotModel CreateExponentialDistribution(double mean = 1, int n = 10000)
+        public static PlotModel CreateExponentialDistribution(bool logarithmicYAxis = false, double mean = 1, int n = 10000)
         {
-            var model = new PlotModel { Title = "Exponential Distribution", Subtitle = "Uniformly distributed bins (" + n + " samples)" };
-            model.Axes.Add(new LinearAxis { Position = AxisPosition.Left, Title = "Frequency" });
+            var model = new PlotModel { Title = logarithmicYAxis ? "Exponential Distribution (logarithmic)" : "Exponential Distribution", Subtitle = "Uniformly distributed bins (" + n + " samples)" };
+            model.Axes.Add(
+                logarithmicYAxis ? 
+                    (Axis)new LogarithmicAxis { Position = AxisPosition.Left, Title = "Frequency"} : 
+                    new LinearAxis { Position = AxisPosition.Left, Title = "Frequency" });
             model.Axes.Add(new LinearAxis { Position = AxisPosition.Bottom, Title = "x" });
 
             Random rnd = new Random(1);

Source/OxyPlot/Series/HistogramSeries.cs

@@ -244,8 +244,16 @@ protected internal void UpdateMaxMinXY()
             {
                 this.MinX = Math.Min(this.ActualItems.Min(r => r.RangeStart), this.ActualItems.Min(r => r.RangeEnd));
                 this.MaxX = Math.Max(this.ActualItems.Max(r => r.RangeStart), this.ActualItems.Max(r => r.RangeEnd));
-                this.MinY = Math.Min(this.ActualItems.Min(r => 0), this.ActualItems.Min(r => r.Height));
-                this.MaxY = Math.Max(this.ActualItems.Max(r => 0), this.ActualItems.Max(r => r.Height));
+                if (this.YAxis.IsLogarithmic())
+                {
+                    this.MinY = Math.Max(this.ActualItems.Min(r => r.Height), double.Epsilon);
+                    this.MaxY = Math.Max(this.ActualItems.Max(r => r.Height), double.Epsilon);
+                }
+                else
+                {
+                    this.MinY = Math.Min(this.ActualItems.Min(r => 0), this.ActualItems.Min(r => r.Height));
+                    this.MaxY = Math.Max(this.ActualItems.Max(r => 0), this.ActualItems.Max(r => r.Height));
+                }
             }
         }
 
@@ -295,10 +303,15 @@ protected void RenderBins(IRenderContext rc, ICollection<HistogramItem> items)
         {
             foreach (var item in items)
             {
+                if (this.YAxis.IsLogarithmic() && !this.YAxis.IsValidValue(item.Height))
+                {
+                    continue;
+                }
+
                 var actualFillColor = this.GetItemFillColor(item);
 
                 // transform the data points to screen points
-                var p1 = this.Transform(item.RangeStart, 0);
+                var p1 = this.Transform(item.RangeStart, this.YAxis.IsLogarithmic() ? double.Epsilon : 0);
                 var p2 = this.Transform(item.RangeEnd, item.Height);
 
                 var rectrect = new OxyRect(p1, p2);