In general, I find data-to-viz.com a great learning tool for graphs.
There is also a great section on stacked areas here: https://www.data-to-viz.com/graph/stackedarea.html

Unfortunately, there is no article on the waterfall chart, this is often considered a variation of the stacked bar chart, but I found this resource which can be useful: https://msktc.org/lib/docs/KT_Toolkit/Charts_and_Graphs/Charts_Waterfall_Charts_508c.pdf

Happy to walk you through these!

Best Practice Strategy/Resources

1) Practice in RocketBlocks

2) Practice in casebooks. LabCo and Enerforce are two of my favorite chart-based cases.

3) Read the Economist (especially the graphic detail) 

4) Ask case partners to focus particularly on your chart-reading skills (i.e. by providing you with cases with many charts) - Bain and Deloitte cases tend to be chart heavy

5) Get a coach. For example, I run a number of exhibit/chart interpretation training programs for other academies/learning centers. Feel free to reach out to get the same training!

In terms of being able to interpret charts/graphs effectively

1) Read the title - and understand it

2) Read the legends - and understand them

3) Remind yourself of the objective / hypothesis in the case, to see where this might fit

4) Find the differences - where does the line graph plummet or spike? Which column is a lot smaller or bigger than the others? Where does change occur? The differences are what matter

5) Talk outloud while interpreting - first, it helps you think and process your thoughts, second, it lets the interviewer provide guidance and course correct if needed.

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Yes, there are several resources available online that can help you practice reading stacked area charts and waterfall charts.

For stacked area charts, you can check out websites like Data-to-viz and Flourish, which provide interactive examples of stacked area charts that you can explore and analyze. You can also search for specific datasets on sites like Kaggle and create your own stacked area charts using tools like Excel or Tableau.

For waterfall charts, you can check out Chartio, which provides an interactive guide on how to read and interpret waterfall charts. You can also search for specific datasets on sites like Github and create your own waterfall charts using tools like Excel or Google Sheets drift boss.

In addition to these resources, it's always a good idea to practice reading charts and graphs in general, as this will help you develop your data literacy skills and make it easier to interpret more complex visualizations like stacked area and waterfall charts.

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Have a look at this question from the past

https://www.preplounge.com/en/consulting-forum/second-order-insights-11693