Add native support for stacked charts

[stsdc]

I'm using stacked chart to display CPU usage and I was wondering if would it be possible to add this as a native feature?

https://github.com/stsdc/monitor/blob/b69fdad44329b66ce8ee479af9f64f6c98f183de/src/Views/SystemView/SystemCPUView.vala#L94-L107

[lcallarec]

Yes, I think I can do that, it's pretty close to stacked bar feature.