drill down based on legends

For me it's not about avoiding two charts but reducing clutter in the 2nd level chart.

e.g. Assume Product Category contains 2 categories of vehicles, 4 wheeled and 2 wheeled, and the Product Sub Category contains for 4 wheeled vehicles trucks, cars and vans and for the 2 wheeled vehicles scooters, sports bikes, cruisers and dirt bikes.

Selecting the 2 wheeled vehicles on the Product Category filter dynamically updates the Product Sub Category filter and chart to only show the scooters, sports bikes, cruisers and dirt bikes without the clutter of the 4 wheeled vehicles.

Like I said before it's an alternative solution. I would still prefer to drill down on the legend, but until then this works for me.

If you want just one chart, another solution is to use a dynamic switch measure, where the filter dynamically changes the measure that is displayed on a single chart. Basically one creates measures for each item to be filtered in the 1st and 2nd levels and then one uses the filter to select which of those measures is displayed in the chart.

https://community.powerbi.com/t5/Quick-Measures-Gallery/Dynamic-Measures-Titles-Using-SWITCH/td-p/200718
https://visualbi.com/blogs/microsoft/powerbi/dynamic-measure-selection-power-bi/