Ah, OK. I see what's happened.
I'd made an error in the title box of the original chart (it said 12 hour sample range, but it was actually 6 hour) so I fixed up the chart and modified my post, removing the wrong image and adding the right one. Unfortunately, before I had done this, Dave picked up the URL for the full image from the wrong one. When I changed the image, the forum software obviously deleted the original, so the link was broken. I could still see it because it was still in my cache. Once I cleared my cache I, too, was unable to view it.
Mea culpa Apologies.