No surprises here.
Unions give to democrats who support them after getting elected, its a vicious cycle. And many of them are government unions using tax payer money, go figure.
Big corporations always stay on the fence as they do not want to be on one side if the other side gets the reign.
All that chart shows is that unions give more to Democrats, which is not surprising. But there is no way that I'm going to believe that unions give orders of magnitude more money than corporations. I'm not a campaign finance expert but what I would guess that this is showing that there is some loophole in transparency laws that is being exploited by corps, or that this chart isn't capturing some important source of money.
I get your skepticism, but are your really going to give more weight to the theoretical omitted data in your mind just to quell your own cognitive dissonance?
There are different ways money are pooled and complicated reporting requirements. If you think Republicans are just peachy than you are an idiot.