The key idea is to float a semi transparent worksheet on top of the dashboard, where a help text box is strategically placed on top of each chart. This way, we can explain how to view each chart and what data points are important, etc. This worksheet is collapsible by a show/hide button. 

Below I would like to show how this worksheet can be constructed.

1. Sheet with a single data mark.

2. Create an show/hide button

• Go to the target dashboard
• Drag a floating vertical container to the dashboard, making it cover all the area of interest.
• Drag the Single Null Mark sheet and drop it into the above container. Hide the sheet title.
• Create an open/close button for the container and place the button at the top-right corner.

3. Add annotations

• Format the sheet background opacity as 70% in the layout manager             
• Select area annotations and place them anywhere of interest. 
• Write help text and format it to highlight important messages.  
• The text can serve as functional guide and/or insight guide.

Here is an example. Feel free to download the workbook and explore. Click the "i" button at the top-right corner to view the in-place help.