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();
@@ -96,4 +102,4 @@ namespace Spectre.Console
             return ((IRenderable)grid).Render(context, width);
         }
     }
-}
+}