I’m having trouble loading the reference file for an IT8.7/2 card in darktable-chart.
I tried multiple reference files (i.e. ones that can be found at http://www.targets.coloraid.de) to check what works and the only kind that actually registers and makes the grid and values appear in the tool is ColorCheckerPassport.cht (found in an article about profiling a camera here at pixls.us).
OS: Windows 10
Target: IT8.7/2 from www.greywhitebalancecolourcard.co.uk