Two new graphs created, discrete, where all discretionary expenses are
plotted, and average, for the yearly average. This later one will be
moved to report form.
Health expenses separated from required. Commute moved from fixed to
required expenses.
New by_month_and_category functions added to transactions.py.
Moves all graph and matplotlib dependencies to graph.py.
Moves Transactions and Categories methods outside the class for better
visibility.
Variable expenses renamed to required.
transactions.py now has by_year, by_month, by_categories in global scope
and slightly improved perfomance.
