For stacked series, adds support for last value in series to be considered as total

[georgepauna]

In my use case, I have a general power meter, and several individual power meters. I wanted to be able to show all values as stacked, but also have a "unknown power consumption", which needs to be calculated as the difference between the overall consumption and the sum of the other series. I'm very sorry for how the commits look, I am VERY new to git + github, this is actually my first time working with them. If you think this is a useful feature, this is how my graph looks:

If you like this, please only take the changes in [apexcharts-card.ts], once again sorry for the mess :( The grey series is calculated as I explained above.