diff --git a/src/Chart.vala b/src/Chart.vala
index afc61ac..1d3aa5c 100644
--- a/src/Chart.vala
+++ b/src/Chart.vala
@@ -52,7 +52,10 @@ namespace CairoChart {
 		public Cairo.Rectangle zoom = Cairo.Rectangle()
 		                              { x = 0, y = 0, width = 1, height = 1 };
 
-		public int zoom_first_show { get; protected set; default = 0; }
+		/**
+		 * 1'st shown series index in zoom area.
+		 */
+		public int zoom_1st_idx { get; protected set; default = 0; }
 
 		public double title_width { get; protected set; default = 0.0; }
 		public double title_height { get; protected set; default = 0.0; }
@@ -103,7 +106,7 @@ namespace CairoChart {
 			chart.title_indent = this.title_indent;
 			chart.title_width = this.title_width;
 			chart.pos = this.pos;
-			chart.zoom_first_show = this.zoom_first_show;
+			chart.zoom_1st_idx = this.zoom_1st_idx;
 			return chart;
 		}
 
@@ -241,10 +244,10 @@ namespace CairoChart {
 				}
 			}
 
-			zoom_first_show = 0;
+			zoom_1st_idx = 0;
 			for (var si = 0, max_i = series.length; si < max_i; ++si)
 				if (series[si].zoom_show) {
-					zoom_first_show = si;
+					zoom_1st_idx = si;
 					break;
 				}
 			var new_zoom = zoom;
@@ -270,7 +273,7 @@ namespace CairoChart {
 				s.place.zoom_y_max = s.place.y_max;
 			}
 			zoom = Cairo.Rectangle() { x = 0, y = 0, width = 1, height = 1 };
-			zoom_first_show = 0;
+			zoom_1st_idx = 0;
 		}
 		public virtual void move (Point delta) {
 			var d = delta;
diff --git a/src/Cursor.vala b/src/Cursor.vala
index 6abb2ae..63c57fc 100644
--- a/src/Cursor.vala
+++ b/src/Cursor.vala
@@ -283,7 +283,7 @@ namespace CairoChart {
 
 					// show joint X value
 					if (chart.joint_x) {
-						var s = chart.series[chart.zoom_first_show];
+						var s = chart.series[chart.zoom_1st_idx];
 						var x = s.get_real_x(chart.rel2scr_x(c.x));
 						string text = "", time_text = "";
 						switch (s.axis_x.type) {
@@ -341,7 +341,7 @@ namespace CairoChart {
 
 					// show joint Y value
 					if (chart.joint_y) {
-						var s = chart.series[chart.zoom_first_show];
+						var s = chart.series[chart.zoom_1st_idx];
 						var y = s.get_real_y(chart.rel2scr_y(c.y));
 						var text_t = new Text(s.axis_y.format.printf((LongDouble)y, s.axis_y.font_style));
 						var print_y = s.compact_rec_y_pos (y, text_t);
@@ -430,12 +430,12 @@ namespace CairoChart {
 			if (chart.series.length == 0) return false;
 			if (list.length() + (is_cursor_active ? 1 : 0) != 2) return false;
 			if (chart.joint_x && cursor_style.orientation == Orientation.VERTICAL) {
-				Float128 val1 = chart.series[chart.zoom_first_show].get_real_x(chart.rel2scr_x(list.nth_data(0).x));
+				Float128 val1 = chart.series[chart.zoom_1st_idx].get_real_x(chart.rel2scr_x(list.nth_data(0).x));
 				Float128 val2 = 0;
 				if (is_cursor_active)
-					val2 = chart.series[chart.zoom_first_show].get_real_x(chart.rel2scr_x(active_cursor.x));
+					val2 = chart.series[chart.zoom_1st_idx].get_real_x(chart.rel2scr_x(active_cursor.x));
 				else
-					val2 = chart.series[chart.zoom_first_show].get_real_x(chart.rel2scr_x(list.nth_data(1).x));
+					val2 = chart.series[chart.zoom_1st_idx].get_real_x(chart.rel2scr_x(list.nth_data(1).x));
 				if (val2 > val1)
 					delta = val2 - val1;
 				else
@@ -443,12 +443,12 @@ namespace CairoChart {
 				return true;
 			}
 			if (chart.joint_y && cursor_style.orientation == Orientation.HORIZONTAL) {
-				Float128 val1 = chart.series[chart.zoom_first_show].get_real_y(chart.rel2scr_y(list.nth_data(0).y));
+				Float128 val1 = chart.series[chart.zoom_1st_idx].get_real_y(chart.rel2scr_y(list.nth_data(0).y));
 				Float128 val2 = 0;
 				if (is_cursor_active)
-					val2 = chart.series[chart.zoom_first_show].get_real_y(chart.rel2scr_y(active_cursor.y));
+					val2 = chart.series[chart.zoom_1st_idx].get_real_y(chart.rel2scr_y(active_cursor.y));
 				else
-					val2 = chart.series[chart.zoom_first_show].get_real_y(chart.rel2scr_y(list.nth_data(1).y));
+					val2 = chart.series[chart.zoom_1st_idx].get_real_y(chart.rel2scr_y(list.nth_data(1).y));
 				if (val2 > val1)
 					delta = val2 - val1;
 				else
@@ -462,7 +462,7 @@ namespace CairoChart {
 			Float128 delta = 0.0;
 			if (!get_cursors_delta(chart, out delta)) return "";
 			var str = "";
-			var s = chart.series[chart.zoom_first_show];
+			var s = chart.series[chart.zoom_1st_idx];
 			if (chart.joint_x)
 				switch (s.axis_x.type) {
 				case Axis.Type.NUMBERS:
diff --git a/src/Series.vala b/src/Series.vala
index 03233e0..ae16215 100644
--- a/src/Series.vala
+++ b/src/Series.vala
@@ -131,7 +131,7 @@ namespace CairoChart {
 				s.join_relative_y_axes (si, true, ref max_rec_width, ref max_rec_height, ref max_font_indent, ref max_axis_font_width, ref nskip);
 
 			// for 4.2. Cursor values for joint X axis
-			if (si == chart.zoom_first_show && chart.cursors.cursors_crossings.length != 0) {
+			if (si == chart.zoom_1st_idx && chart.cursors.cursors_crossings.length != 0) {
 				switch (chart.cursors.cursor_style.orientation) {
 				case Cursors.Orientation.VERTICAL:
 					if (is_x && chart.joint_x)
@@ -149,12 +149,12 @@ namespace CairoChart {
 					break;
 				}
 			}
-			if (is_x && (!chart.joint_x || si == chart.zoom_first_show))
+			if (is_x && (!chart.joint_x || si == chart.zoom_1st_idx))
 				switch (axis.position) {
 				case Axis.Position.LOW: chart.plot_y_max -= max_rec_height + max_font_indent + max_axis_font_height; break;
 				case Axis.Position.HIGH: chart.plot_y_min += max_rec_height + max_font_indent + max_axis_font_height; break;
 				}
-			if (!is_x && (!chart.joint_y || si == chart.zoom_first_show))
+			if (!is_x && (!chart.joint_y || si == chart.zoom_1st_idx))
 				switch (s.axis_y.position) {
 				case Axis.Position.LOW: chart.plot_x_min += max_rec_width + max_font_indent + max_axis_font_width; break;
 				case Axis.Position.HIGH: chart.plot_x_max -= max_rec_width + max_font_indent + max_axis_font_width; break;
@@ -319,7 +319,7 @@ namespace CairoChart {
 		public virtual void draw_horizontal_axis (int si, ref int nskip) {
 			var s = chart.series[si];
 			if (!s.zoom_show) return;
-			if (chart.joint_x && si != chart.zoom_first_show) return;
+			if (chart.joint_x && si != chart.zoom_1st_idx) return;
 
 			// 1. Detect max record width/height by axis.nrecords equally selected points using format.
 			double max_rec_width, max_rec_height;
@@ -444,7 +444,7 @@ namespace CairoChart {
 		public virtual void draw_vertical_axis (int si, ref int nskip) {
 			var s = chart.series[si];
 			if (!s.zoom_show) return;
-			if (chart.joint_y && si != chart.zoom_first_show) return;
+			if (chart.joint_y && si != chart.zoom_1st_idx) return;
 			// 1. Detect max record width/height by axis.nrecords equally selected points using format.
 			double max_rec_width, max_rec_height;
 			s.axis_y.calc_rec_sizes (chart, out max_rec_width, out max_rec_height, false);