Add a legend / key to your diagram simply by adding a Product to the screen.
Steps to Display Legend:
Add at least one Product to the canvas
Tap the first icon in the top right corner
Tap Create a Legend Table
Steps to Update Legend:
Tap the first icon in the top right corner
Tap Refresh a Legend Table or Refresh Legend Table
Change Legend Display Settings:
Tap the Legend Settings icon in the lower left corner of the canvas
There you can change the Legend title and the number of columns
Alternatively you can resizing the legend itself