Below is the plot of all parties, sauf the Bill and Ben party and Legalise Cannabis. B&B would skew the stats because unlike many of the other parties they host a popular TV show, and Legalise Cannabis (to my knowledge) are more of a single-issue lobby group than a political party, so perhaps don't target actual votes with their advertisement in quite the same way as the others.
There's not much to see here. Unfortunately the data is so spread out that it's difficult to make any conclusions. The r squared for the linear regression seems to be pretty high, but that must be because of the lower values, because visually the residuals look pretty large for the more successful parties other than the one the trendline is jabbing in the middle-right, which is Labour.
This is basically just the minority parties - by which I mean super-minority, i.e. not even in Parliament (or barely, as with the Progressives and United Future). There is a much clearer linear correlation here. It's been said, but correlation doesn't imply causation. But it is clear that higher spending is pretty well correlated with higher votes. A lot of people just plainly assume that money buys votes, hence why we need equalised political funding, control on donations, or what have you. I tend to think that people are a bit more rational than that, and that a better explanation for this data is that people tend to donate to the parties they vote for.In case you're too impressed by my high R squared values, I'll finish with this graph.
Correlation is easy to get.Incidentally the maximum of this graph is visible on the chart, it is at (2.87,1.56). This means that (according to my perfectly correlative model) the optimal amount of spending is $2.87m, and it will get you 1.56m votes, which at the 2008 election would have seen you earn 67% of the votes.
I can't believe I'm giving this stuff away for free.