Add support custom max value for barcharts (#545)

src/Spectre.Console/Extensions/BarChartExtensions.cs

@@ -238,5 +238,22 @@ namespace Spectre.Console
             chart.LabelAlignment = Justify.Right;
             return chart;
         }
+
+        /// <summary>
+        /// Sets the max fixed value for the chart.
+        /// </summary>
+        /// <param name="chart">The bar chart.</param>
+        /// <param name="maxValue">Max value for the chart.</param>
+        /// <returns>The same instance so that multiple calls can be chained.</returns>
+        public static BarChart WithMaxValue(this BarChart chart, double maxValue)
+        {
+            if (chart is null)
+            {
+                throw new ArgumentNullException(nameof(chart));
+            }
+
+            chart.MaxValue = maxValue;
+            return chart;
+        }
     }
 }

src/Spectre.Console/Widgets/Charts/BarChart.cs

@@ -43,6 +43,12 @@ namespace Spectre.Console
         /// <remarks>Defaults to invariant culture.</remarks>
         public CultureInfo? Culture { get; set; }
 
+        /// <summary>
+        /// Gets or sets the fixed max value for a bar chart.
+        /// </summary>
+        /// <remarks>Defaults to null, which corresponds to largest value in chart.</remarks>
+        public double? MaxValue { get; set; }
+
         /// <summary>
         /// Initializes a new instance of the <see cref="BarChart"/> class.
         /// </summary>
@@ -62,7 +68,7 @@ namespace Spectre.Console
         protected override IEnumerable<Segment> Render(RenderContext context, int maxWidth)
         {
             var width = Math.Min(Width ?? maxWidth, maxWidth);
-            var maxValue = Data.Max(item => item.Value);
+            var maxValue = Math.Max(MaxValue ?? 0d, Data.Max(item => item.Value));
 
             var grid = new Grid();
             grid.Collapse();

test/Spectre.Console.Tests/Expectations/Widgets/BarChart/Fixed_Max_Value.Output.verified.txt

@@ -0,0 +1,4 @@
+                          Number of fruits                  
+ Apple  ███ 12                                              
+Orange  █████████████████████████ 54                        
+Banana  ██████████████ 33                                   

test/Spectre.Console.Tests/Unit/Widgets/BarChartTests.cs

@@ -47,5 +47,26 @@ namespace Spectre.Console.Tests.Unit
             // Then
             await Verifier.Verify(console.Output);
         }
+        
+        [Fact]
+        [Expectation("Fixed_Max_Value")]
+        public async Task Should_Render_Correctly_3()
+        {
+            // Given
+            var console = new TestConsole();
+
+            // When
+            console.Write(new BarChart()
+                .Width(60)
+                .WithMaxValue(100)
+                .Label("Number of fruits")
+                .AddItem("Apple", 12)
+                .AddItem("Orange", 54)
+                .AddItem("Banana", 33));
+
+            // Then
+            await Verifier.Verify(console.Output);
+        }
+
     }
 }