Just a quick warning in case anyone is being taken in as we speak by James Mackenzie's latest attempts on behalf of the Greens to claim that only they can stop UKIP in Scotland. YouGov's "final Scottish poll" of which he speaks is no such thing - it's a subsample of a GB-wide poll. Yes, it's a larger than usual subsample, but it's still a subsample. So it's not possible to claim as James does that "the margin of error is bigger than with a normal 1000 person poll (up to 4.2%, rather than around 3%), but even so..." The margin of error is in fact incalculable, because the figures almost certainly won't have been weighted properly - ie. they'll only have been weighted on a GB-wide basis. What weightings were applied will have been based on YouGov's party ID system, rather than the 2011 vote recall they use for full-scale Scottish polls. That almost always leads to SNP identifying respondents being downweighted massively. In this poll, 165 SNP and Plaid Cymru identifiers were downweighted to count as just 104 'virtual' respondents.
The facts remain the same - not a single full-scale Scottish poll in this campaign has shown the Greens even close to winning a seat today. Most polls (including from YouGov) show that the SNP are best placed to stop UKIP, although there's certainly still a chance that Labour or the Tories could be the ones to pull it off. But if you vote Green you're not "voting tactically" at all - you're effectively abstaining on the question of whether UKIP should have a Scottish seat or not.
Maggie Chapman is a very fine candidate (and an asset for the Yes campaign), and if your number one priority is to have her as an MEP representing Scotland, then by all means vote Green - even though the chances of success look very slim indeed. But just be aware that it isn't going to stop David Coburn or UKIP.