Sum up series in a graph.

Hello guys.
So this is my first post.

I have crated a chart with 4 different series. X axis is a value (in this case money), and Y axis is a date.
The four different series comes from 4 different tables.

Each serie stands for money on a account shown over date and total amount of money on each date.
So I have four different accounts here that I am following in this graph with four series.

So what I need help with;
I want to be able to see a 5th serie/line in my graph which would me the sum of all the other four graphs.
I want to see how much cash I have on all the accounts, together, buy the given date.

I have figured out how to do this in the complicated way, which would be to make a 5th table and then make my 5th serie.
In order to do so I had to take all the four tables and put the dates in right order so that the value will be correct. This is annoying.

How can I directly sum up my four series with each other to create the 5th one?

Difficult to explain, but here's a picture that shows what I made.
http://postimg.org/image/lo9i8kpi3/full/

Just ask me whatever you need!

Thanks in advance