To fix issue #100 convert dict comprehension to generator expression for 2.6

to fix issue #100

https://github.com/yhat/ggplot/issues/100

I am a n00b to ggplot, tried to play ggplot on an old machine with python 2.6 manually fixed the issue, my initial tests are ok-ish

Please kindly consider this merge, I am sorry if I broke something I am unaware of.