"you can probably display each subroutine call as a pyramid of the calls that happen inside"
Oops, just after I posted this I realized that the "calls that happen inside" a subroutine may vary depending on the program state or arguments. If these separate possibilities are not shown as separate locations in the fractal, then zooming in won't be sufficient to narrow down these parameters in the view. If they are shown as separate locations, then a mere 8-bit parameter will shrink each sub-pyramid by a factor of 256.
It's likely the zooming interface will need to be accompanied by a timeline scrubber or tableau view to visualize number-like parameters.
Pointer-like parameters would get out of hand either way, let alone strings, lists, trees, and functions.