But here's a monthly average of public polls from Britain Elects, FWIW:
The Labour/Conservative crossover point reflects the most reliable recent poll we have - the general election, after which some pollsters may have revised their methods. Prior to that, it's worth bearing in mind that the parties' private polls showed the crossover actually having taken place following the 2014 conference season, with the Tories consistently thereafter polling 4-6 points higher than the public polls.
One thing the graph doesn't tell us is the margin of error. Bearing that in mind, it doesn't (yet, anyway) show a catastrophic decline in Labour voting intentions after Corbyn became leader. Labour's worst dip came immediately after the election to just below 30% in June, when Labour was in even worse disarray than usual as the messy leadership contest and pre-vote in-fighting was taking hold.
It's open to argument whether the relatively small dip October-November (which may be within the margin of error anyway) is because of Corbyn or because of the vocal anti-Corbynites. It may also reflect a return to norm after a brief and not particularly stellar Corbyn honeymoon period. We obviously have no way of knowing what this graph would look like if another of the contenders had won the Labour leadership - could be worse, could be better.
The other thing such a broad picture (with all the caveats mentioned above) doesn't show is what's happening at constituency level. Given the role of key marginals in the last election, that's crucial. But the next general election is so far off anyway - literally anything could happen. There are also quite a few Tory chickens flapping their wings as they prepare to come home to roost.
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