A recent problem with graphiz collaboration diagrams no rendering in 'readthedocs'

[The9Cat]

We have successfully used breathe to integrate our C++ class documentation from doxygen into our Sphinx readthedocs online manual. However, sometime in the last few months, the graphiz renderings have ceased to work. There have been no changes to repository during that time, so something else must have changed on the readthedocs side. The rendered html still works correctly on a local build using Ubuntu 24.04 packages for Sphinx and Breathe, but the same source built by readthedocs shows stuff like

digraph { graph [bgcolor="#00000000"] node [shape=rectangle style=filled fillcolor="#FFFFFF" font=Helvetica padding=2] edge [color="#1414CE"] "2" [label="cxxopts::Value" tooltip="cxxopts::Value"] "1" [label="cxxopts::values::abstract_value< T >" tooltip="cxxopts::values::abstract_value< T >" fillcolor="#BFBFBF"] "4" [label="cxxopts::values::standard_value< T >" tooltip="cxxopts::values::standard_value< T >"] "3" [label="std::enable_shared_from_this< Value >" tooltip="std::enable_shared_from_this< Value >"] "2" -> "3" [dir=forward tooltip="public-inheritance"] "1" -> "2" [dir=forward tooltip="public-inheritance"] "4" -> "1" [dir=forward tooltip="public-inheritance"] }

rather than the rendered graphic element.

Does anyone have any suggestions?