Add support for bar charts

Closes #103

src/Spectre.Console/Widgets/BarChartItem.cs

@@ -0,0 +1,38 @@
+using System;
+
+namespace Spectre.Console
+{
+    /// <summary>
+    /// An item that's shown in a bar chart.
+    /// </summary>
+    public sealed class BarChartItem : IBarChartItem
+    {
+        /// <summary>
+        /// Gets the item label.
+        /// </summary>
+        public string Label { get; }
+
+        /// <summary>
+        /// Gets the item value.
+        /// </summary>
+        public double Value { get; }
+
+        /// <summary>
+        /// Gets the item color.
+        /// </summary>
+        public Color? Color { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="BarChartItem"/> class.
+        /// </summary>
+        /// <param name="label">The item label.</param>
+        /// <param name="value">The item value.</param>
+        /// <param name="color">The item color.</param>
+        public BarChartItem(string label, double value, Color? color = null)
+        {
+            Label = label ?? throw new ArgumentNullException(nameof(label));
+            Value = value;
+            Color = color;
+        }
+    }
+}