diff --git a/src/Area.vala b/src/Area.vala
index 46dfd01..edab98f 100644
--- a/src/Area.vala
+++ b/src/Area.vala
@@ -28,7 +28,7 @@ namespace CairoChart {
 		/**
 		 * ``Area`` width.
 		 */
-		public double width {
+		public virtual double width {
 			get {
 				return x1 - x0;
 			}
@@ -40,7 +40,7 @@ namespace CairoChart {
 		/**
 		 * ``Area`` height.
 		 */
-		public double height {
+		public virtual double height {
 			get {
 				return y1 - y0;
 			}
@@ -107,7 +107,7 @@ namespace CairoChart {
 		/**
 		 * Gets a copy of the ``Chart``.
 		 */
-		public Area copy () {
+		public virtual Area copy () {
 			return new Area.with_area(this);
 		}
 	}
diff --git a/src/Chart.vala b/src/Chart.vala
index 6d55e99..e52a57d 100644
--- a/src/Chart.vala
+++ b/src/Chart.vala
@@ -58,17 +58,17 @@ namespace CairoChart {
 		/**
 		 * Index of the 1'st shown series in a zoomed area.
 		 */
-		public int zoom_1st_idx { get; protected set; default = 0; }
+		public virtual int zoom_1st_idx { get; protected set; default = 0; }
 
 		/**
 		 * Joint/common X axes or not.
 		 */
-		public bool joint_x { get; protected set; default = false; }
+		public virtual bool joint_x { get; protected set; default = false; }
 
 		/**
 		 * Joint/common Y axes or not.
 		 */
-		public bool joint_y { get; protected set; default = false; }
+		public virtual bool joint_y { get; protected set; default = false; }
 
 		/**
 		 * Joint/common {@link Axis} ``Color``.
@@ -83,13 +83,13 @@ namespace CairoChart {
 		/**
 		 * ``Chart`` cursors.
 		 */
-		public Cursors cursors { get; protected set; default = null; }
+		public virtual Cursors cursors { get; protected set; default = null; }
 
 		/**
 		 * Set paint color for further drawing.
 		 */
-		public Color color {
-			private get { return Color(); }
+		public virtual Color color {
+			protected get { return Color(); }
 			set { ctx.set_source_rgba (value.red, value.green, value.blue, value.alpha); }
 			default = Color();
 		}
@@ -106,7 +106,7 @@ namespace CairoChart {
 		/**
 		 * Gets a copy of the ``Chart``.
 		 */
-		public Chart copy () {
+		public virtual Chart copy () {
 			var chart = new Chart ();
 			chart.area = this.area.copy();
 			chart.bg_color = this.bg_color;