[FEATURE] Stacked area chart example

[nickhenden]

Preliminary Checks

• I have searched existing issues to ensure this feature hasn't been requested before

Proposed Solution

Add Matplotlib and Plotly example code for a stacked area chart.

Examples:

• Standard area charts in the afcharts cookbook
• Stacked area chart (ONS)

This would be needed for both Matplotlib and Plotly pages. If you are tackling just one of these, please create a specific sub-issue for Matplotlib/Plotly accordingly.