Support multiple time intervals for charts

[kbence]

For charts, it would make sense to support multiple time intervals, if they cover the same amount of time. Then the multiple lines could be put on the same chart - with different colors. There are a few possible ideas how to implement it.

• Using the 7 basic terminal colors. This makes it possible to display up to 7 charts at the same time
• Using the 3 additive colors, add them up on overlapping. This might be nice but it might not make sense.
• Area filled charts with the 3 additive colors and adding them up on overlapping. This might be a nice addition, it should be tested if it's useful.

There's one small problem: whichever solution we choose, the color additions will have a lower resolution (except for classic and block modes). The easiest way is to always choose the last written color for a specific character.