Add vtkChartXY::SetTooltip() method

This adds a new method to the vtkChartXY class which
allows for the default vtkTooltipItem for the chart to
be changed.

Change-Id: If831a73b88ef12db0a6be24200624cbe0ffa1170

Charts/vtkChartXY.cxx

@@ -1077,6 +1077,30 @@ vtkChartLegend* vtkChartXY::GetLegend()
   return this->Legend;
 }
 
+//-----------------------------------------------------------------------------
+void vtkChartXY::SetTooltip(vtkTooltipItem *tooltip)
+{
+  if(tooltip == this->Tooltip)
+    {
+    // nothing to change
+    return;
+    }
+
+  if(this->Tooltip)
+    {
+    // remove current tooltip from scene
+    this->RemoveItem(this->Tooltip);
+    }
+
+  this->Tooltip = tooltip;
+
+  if(this->Tooltip)
+    {
+    // add new tooltip to scene
+    this->AddItem(this->Tooltip);
+    }
+}
+
 //-----------------------------------------------------------------------------
 vtkTooltipItem* vtkChartXY::GetTooltip()
 {

Charts/vtkChartXY.h

@@ -102,6 +102,10 @@ public:
   // Get the vtkChartLegend object that will be displayed by the chart.
   virtual vtkChartLegend* GetLegend();
 
+  // Description:
+  // Set the vtkTooltipItem object that will be displayed by the chart.
+  virtual void SetTooltip(vtkTooltipItem *tooltip);
+
   // Description:
   // Get the vtkTooltipItem object that will be displayed by the chart.
   virtual vtkTooltipItem* GetTooltip();