I have been searching for a working example of a chart inside tooltips of the another chart. I have not been able to find something so far. I am wondering if it is at all possible.
Yes, probably it is not very common but as far as I know it is possible and you just need to append the new D3 SVG drawing into the tooltip with the function
d3.select("#tooltip").append("svg:svg") //Line 252
On this D3 chart tooltip JSFiddle I'm adding a D3 chart to the #tooltip element.