From Mark Hopwood at yesterday's TravelWatch SouthWest:

Purple -
GWR▸ (our
TOC▸ ) issues
Blue - Network Rail issues
Green - TOC on TOC (
SWR» , Cross Country,
TfW▸ , freight, and perhaps others holding up or blocking GWR trains)
I can't help wondering whether the diagram should show all failures to provide what's in the published May to December timetable - as I understand it, the cutoff is a few days ahead so that trains cancelled due to industrial action by GWR and Network Rail employees do not show, and the picture above is a false (massaged) statistic. I think I am happy that trains replaced by alternative services of buses during engineering works planned months in advance don't show up in this chart.