Fix invalidation after hidden (#53)

Author: VisualMelon

Source/OxyPlot.Avalonia/PlotBase.cs

@@ -237,7 +237,6 @@ public void InvalidatePlot(bool updateData = true)
             {
                 if (Interlocked.CompareExchange(ref isUpdateRequired, updateState, currentState) == currentState)
                 {
-                    isUpdateRequired = updateState;
                     BeginInvoke(() => UpdateModel(updateData));
                     break;
                 }
@@ -394,6 +393,7 @@ protected void UpdateModel(bool updateData = true)
         {
             if (Width <= 0 || Height <= 0 || ActualModel == null)
             {
+                isUpdateRequired = 0;
                 return;
             }
 
@@ -403,7 +403,7 @@ protected void UpdateModel(bool updateData = true)
 
                 if (updateState > 0)
                 {
-                    ((IPlotModel)ActualModel).Update(updateState == 2);
+                    ((IPlotModel)ActualModel).Update(updateState == 2 || updateData);
                 }
             }
 
@@ -413,7 +413,6 @@ protected void UpdateModel(bool updateData = true)
                 // After the invalidation, the element will have its layout updated,
                 // which will occur asynchronously unless subsequently forced by UpdateLayout.
                 BeginInvoke(InvalidateArrange);
-                BeginInvoke(InvalidateVisual);
             }
 
         }