Most of methods should be virtual.

src/Axis.vala

@@ -77,7 +77,7 @@ namespace CairoChart {
 		public Line.Style line_style = Line.Style ();
 		public double font_indent = 5;
 
-		public Axis copy () {
+		public virtual Axis copy () {
 			var axis = new Axis ();
 			axis._date_format = this._date_format;
 			axis._dsec_signs = this._dsec_signs;

src/Grid.vala

@@ -8,7 +8,7 @@ namespace CairoChart {
 
 		public Line.Style line_style = Line.Style ();
 
-		public Grid copy () {
+		public virtual Grid copy () {
 			var grid = new Grid ();
 			grid.color = this.color;
 			grid.line_style = this.line_style;

src/Legend.vala

@@ -21,7 +21,7 @@ namespace CairoChart {
 		public double text_vspace = 2.0;
 		public bool show = true;
 
-		public Legend copy () {
+		public virtual Legend copy () {
 			var legend = new Legend ();
 			legend.position = this.position;
 			legend.font_style = this.font_style;

src/Marker.vala

@@ -20,7 +20,7 @@ namespace CairoChart {
 			this.size = size;
 		}
 
-		public Marker copy () {
+		public virtual Marker copy () {
 			return new Marker (type, size);
 		}
 

src/Place.vala

@@ -29,7 +29,7 @@ namespace CairoChart {
 		public double zoom_y_min = 0;
 		public double zoom_y_max = 1;
 
-		public Place copy () {
+		public virtual Place copy () {
 			var place = new Place ();
 			place.x_min = this.x_min;
 			place.x_max = this.x_max;

src/Series.vala

@@ -41,7 +41,7 @@ namespace CairoChart {
 
 		public bool zoom_show = true;
 
-		public Series copy () {
+		public virtual Series copy () {
 			var series = new Series ();
 			series._color = this._color;
 			series.axis_x = this.axis_x.copy ();

src/Text.vala

@@ -1,11 +1,10 @@
 namespace CairoChart {
-	[Compact]
 	public class Text {
 		public string text = "";
 		public Font.Style style = Font.Style ();
 		public Color color = Color();
 
-		public Cairo.TextExtents get_extents (Cairo.Context context) {
+		public virtual Cairo.TextExtents get_extents (Cairo.Context context) {
 			context.select_font_face (style.family, style.slant, style.weight);
 			context.set_font_size (style.size);
 			Cairo.TextExtents extents;
@@ -13,7 +12,7 @@ namespace CairoChart {
 			return extents;
 		}
 
-		public double get_width (Cairo.Context context) {
+		public virtual double get_width (Cairo.Context context) {
 			var extents = get_extents (context);
 			switch (style.orientation) {
 			case Font.Orientation.HORIZONTAL: return extents.width;
@@ -22,7 +21,7 @@ namespace CairoChart {
 			}
 		}
 
-		public double get_height (Cairo.Context context) {
+		public virtual double get_height (Cairo.Context context) {
 			var extents = get_extents (context);
 			switch (style.orientation) {
 			case Font.Orientation.HORIZONTAL: return extents.height;
@@ -36,7 +35,7 @@ namespace CairoChart {
 			double height;
 		}
 
-		public Size get_size (Cairo.Context context) {
+		public virtual Size get_size (Cairo.Context context) {
 			var sz = Size();
 			var extents = get_extents (context);
 			switch (style.orientation) {
@@ -52,7 +51,7 @@ namespace CairoChart {
 			return sz;
 		}
 
-		public void show (Cairo.Context context) {
+		public virtual void show (Cairo.Context context) {
 			context.select_font_face(style.family,
 			                         style.slant,
 			                         style.weight);
@@ -74,7 +73,7 @@ namespace CairoChart {
 			this.color = color;
 		}
 
-		public Text copy () {
+		public virtual Text copy () {
 			var text = new Text ();
 			text.text = this.text;
 			text.style = this.style;