The tooltip for each kitten shows its name and photo.
When you zoom in or out the effective size of each Node is kept constant by changing its GraphObject.scale.
A tooltip is an Adornment that is shown when the mouse hovers over an object that has its GraphObject.toolTip set. The tooltip part is bound to the same data as the part itself.
It is typical to implement a tooltip as a "ToolTip" Panel holding a TextBlock or a Panel of TextBlocks and other objects. Each "ToolTip" is just an "Auto" Panel Adornment that is shadowed, and where the border is a rectangular Shape with a light gray fill. However you can implement the tooltip as any arbitrarily complicated Adornment.
More information can be found in the GoJS Intro.