area chart combined with metrics

[grain209]

Let's add a new chart type like the one below:

"Views Last 24h" from
https://keen.github.io/auto-collector-dashboard/

It contains 3 things:

1. count this_24_hours
2. count this_48_hours - this_24_hours
3. count this_24_hours with interval hourly

[apryka]

added in version 3.5.0