Charts make it possible to plot multiple variables as your diagram runs, to visualise possible outcomes, and help you monitor certain parameters either standalone, or in relationship to others.
Here's an example of plotting 2 variables. In a basic diagram representing a running game, we are plotting Coins earned by a player during a run and the Difficulty Factor that influences the player's Chance of Error (aka crashing into obstacles).
In the video above, Machinations simulates one run, and plots the evolution over time steps of the 2 Pools. Notice this run took over 100 seconds (time steps). By the time the run was over, the player accumulated 272 Coins and reached a Difficulty Factor of 51.
A quick look at the chart could tell us that we may possibly be handing out Coins at a too high rate. From here on, you can start iterating, and answering questions like: Is it an early or a late run issue? Do I have enough content for my player to sink the Coins, or can the rate of Coin collecting result in inflation? How does this chart look like when I introduce power-ups? Should I increase the Difficulty Factor?