Resolve "Knobs aren't reactive for chart components"

What does this MR do

Currently, none of the chart-related storybook entries have their knobs working correctly.

The values can be changed, but updates don't reflect in the rendered result.

This commit changes the affected stories to use the pattern suggested by the storybook-knobs documentation to fix the behavior.

How to replicate the issue

Go to https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/charts-area-chart--default and try tweaking any of the knobs.

Related to #206 (closed)