how i plot a profit_loss graph

have income,expense, user controller i want to plot a graph of currrent user profit i calculated it in model.
def total_expense
expenses.pluck(:amount).sum
end
def total_incomes
incomes.pluck(:amount).sum
end
def profit_loss
total_incomes - total_expense
end
how i plot a graph of profit_loss its value is not save in data base