The ROOTbook is an interactive computing environment.
Select Executable ROOTbooks. Then you have a choice of Python or C++.
The Notebook has two modes:
Edit Mode: Allows you to insert text into a cell. Pressing Enter or clicking in an input text area / cell switches to Edit Mode. A pencil icon is displayed in the indicator area (top right). Notice that the border around the currently active cell changed color to green.
Command Mode: Binds the keyboard to notebook level actions. Pressing Esc or clicking outside of the input text area takes you back to Command Mode. No icon is displayed in the indicator area. The cell border colour is grey.
Start by clicking in the first input text area. Note you are now in Edit Mode (see screen shot red circle showing edit icon indicator) and the text box border is green.
Press Control+Return to step through the commands in each cell.
You can edit, run the commands again, and see your changes. E.g.
There are further options for changing the display of your histogram e.g. changing y-axis to log scale.