A green electoral trend

The bar chart below, from Green party leaflets, is very interesting. The argument here is that the share of their vote is growing and voting for them is not a lost vote. I have a lot of sympathy for the argument for proportional representation. The 2015 British national election was a joke, with UKIP being a third party in votes but not in seats, and SNP benefiting massively from first past the post to get a monopoly of Scottish seats that is not quite reflected in the vote counts. 

However, this chart poses a few questions for me. Sure, the green vote grew from 2010 to 2015, but was it a one off that cannot be projected into further growth for 2017? 

After all 2015 was the fall of the LibDems, and those voters who felt betrayed by Clegg but hadn't forgiven Labour either could vote Green. The situation of course is very different now, with Labour breaking away entirely from the Blairite past while the LibDems are trying to capitalise on the 48% Remain vote (by repeating the Guardian quote 'Corbyn betrayed us' among other things).
An equally interesting feature of this bar chart is the plateau from 2005 to 2010, which supports the argument that 2015 was an outlier and not the manifestation of an underlying trend. Last decade had very different priorities to the current one; the two big parties had largely taken on-board environmental concerns, the Tories even had a green tree as a logo (the same one that was repainted with the union jack recently), and even though the economic crisis was obvious at the end of the decade, the political priorities took a while to change. So maybe 2010 is the outlier, and the trend is there.
We'll only know once the 2017 results are out!