Well I think ‘savings’ in this context is just the amount left over when you take all of your expenses away from your income. This is straight forward for a personal account with a single income. But its more tricky with a joint account with multiple incomes and outgoings that are not necessarily expenses (e.g. moving money into a personal account).
Oh and I think generally we’re not looking to make any predictions with these graphs, they are mostly retrospective (aside from some of my efforts to doing forecasting)