Fix how series render in VSA Overview "Total time" chart

Summary

Despite summing the total of the average durations of each stage, the series in the VSA Overview "Total time" stacked area chart are overlaying one another rather than stacking cleanly.

What is the expected correct behavior?

The series should stack rather than overlay.

Relevant logs and/or screenshots

Current Behavior	With stack

Possible fixes

Add stack: 'chart' to the chart's options, while ensuring that the null series renders correctly as well.