Stacked line chart support

We should offer support for stacked line charts. These are very useful when you have multiple series, which are different flavors of the same general quantity and so they should be cumulative.

For example consider Throughput, which is a measure of requests per second. With support for multiple series, we would like to offer three series for 2xx, 4xx, and 5xx.

We could simply render these three lines independently, but then it becomes hard to understand the total throughput. Instead, we can use a stacked line chart which shows the contribution of each individual series, as well as making it easy to understand the cumulative value.