Each node contains a Table Panel whose Panel.itemArray is data bound to the "items" property which holds an Array of data objects. That Table Panel has an Panel.itemTemplate which creates a bar (a rectangular Shape) and a TextBlock label for each item. Each bar also has a tooltip showing the value.
For more sophisticated charts within nodes, see the Canvas Charts sample.
The "Table" Panel, Panel.Table, arranges objects in rows and columns. Each object in a Table Panel is put into the cell indexed by the value of GraphObject.row and GraphObject.column. The panel will look at the rows and columns for all of the objects in the panel to determine how many rows and columns the table should have. More information can be found in the GoJS Intro.
It is sometimes useful to display a variable number of elements in a node by data binding to a JavaScript Array. In GoJS, this is simply achieved by binding (or setting) Panel.itemArray. The Panel will create an element in the panel for each value in the Array. More information can be found in the GoJS Intro.
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.