I managed to track this down to the favourite link on my favourite bar. Copying the URL from the item into the address bar gave the right page first time, pressing the icon on the favourite bar gave the wrong graphs. Checking the page source, identical in both cases.
I then generated a new favourite item from the working page, and amazingly this one worked, whilst the original still didn't, despite all the fields in the favourite (including permissions, icon, etc) seemingly being identical.
Life being short, I have deleted the non-working favourite, renames the new one, and all now appears fine.