[FEATURE] Bar chart with negative values 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 bar chart with negative values.

Examples:

• Analysis Function charts guidance
• Analysis Function bar charts guidance: Figure 2
• Analysis Function stacked bar charts guidance: Figure 6b

Example code 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.