no diff / empty record.json if initial screenshots differ

[artm]

I am trying to make a test for a chart widget which seems to render slightly different each time. I was hoping running huxley with with --record and --save-diff would save the diff between the rerecorded and the second run screenshot, but it doesn't. And the record.json is empty so I have to record my getting to the unstable chart again.

I understand I have to create a test page for just that one chart and test only that, so I won't need a complicated interaction to get to it, but wouldn't it be nice to have the diffs/record.json saved anyway?