improved Area Chart error conditions

xchart-demo/src/main/java/com/xeiam/xchart/demo/charts/area/AreaChart02.java

@@ -64,6 +64,10 @@ public class AreaChart02 implements ExampleChart {
     }
     xData.add(10);
     yData.add(null);
+    xData.add(11);
+    yData.add(100);
+    xData.add(12);
+    yData.add(90);
 
     chart.addSeries("a", xData, yData);
 

xchart/src/main/java/com/xeiam/xchart/StyleManager.java

@@ -31,6 +31,9 @@ import com.xeiam.xchart.internal.style.XChartTheme;
  */
 public class StyleManager {
 
+  /**
+   * Note: For Area Charts, the X-Axis data must be in ascending order.
+   */
   public enum ChartType {
 
     Line, Scatter, Area, Bar

xchart/src/main/java/com/xeiam/xchart/internal/chartpart/PlotContentLineChart.java

@@ -194,9 +194,11 @@ public class PlotContentLineChart extends PlotContent {
               path.moveTo(previousX, yBottomOfArea);
               path.lineTo(previousX, previousY);
             }
-
             path.lineTo(xOffset, yOffset);
           }
+          if (xOffset < previousX) {
+            throw new RuntimeException("X-Data must be in ascending order for Area Charts!!!");
+          }
         }
 
         previousX = xOffset;