As, for most people who use both Gas and Electricity, Gas amounts will be more variable, good data visualization practice would be to have Electricity at the bottom and Gas at the top. Why? Because with it this way round, you cannot see at a glance how Electricity bills change each month. If it were the other way round, you would see a mildly changing electricity bill - and you could still see the big seasonal changes in the Gas bills.
Are there any opposing viewpoints?