Update chart component to include 2nd Y-axis variant
When showing a chart containing data sets at different orders of magnitude (e.g. issues closed, 10; issues opened, 10; total issues 1,000), this reduces the size of the visualisations at lower orders of magnitude to a point where they aren't useful.
- Add a variant to our charts component which has a second Y-axis.
- Add guidance to our documentation on using this (potentially confusing) pattern