Sliding legend

From the looks of things, there is something that happens with re-drawing the nav buttons when clicked that seems to ignore any styling or changes applied. I made some attempts in one of my visuals to do something similar in the past (to change the shape of the icons), but found that it was more trouble that it was worth and abandoned my efforts.

If you want to do this in a maintainable way, then I think you'll need to create an idea with the custom visuals team to add support. You can refer to this post in the Custom Visuals Development Discussion Forum for more details.

Regarding your other question, if you can't write DAX in-model, I would suggest requesting the owners of the OLAP cube add in such a measure for you - these tools are designed for such functionality and it is minimal work for them to manage and support. If the OLAP cube is SSAS, then SSAS Tabular uses DAX for its measures; SSAS Multidimensional can achieve the same via MDX. Solving in the semantic model is going to be the way to go.

Not to discourage learning how to develop custom visuals at all - I would personally love to have more people to collaborate with and learn from - but this is a very heavy approach to solving a problem that can be resolved upstream and is going to be several of hours' work from my side to document how such a solution could conceivably work (and much more from yours to develop). I'm a big subscriber to working smarter rather than harder. As it's outside the scope of the original question, I think it probably warrants its own thread if you specifically want to go through this.

However, if I were to do this, then I'd start by working with the standard categorical data mapping that comes with a new visual project, and set my view model hierarchy as measure[] > category[] > value[]. Once I've mapped my distinct measures and categories, as I iterate through the value array, I'd add to the previous value so that each subsequent value in the value array for each measure/category is a cumulative total.