allow string keys in size legend
allow string keys in size legend

=> __main__ test doesn't crash anymore

benslice/ggplot,eco32i/ggplot,ricket1978/ggplot,kmather73/ggplot,udacity/ggplot,ricket1978/ggplot,mizzao/ggplot,Cophy08/ggplot,wllmtrng/ggplot,benslice/ggplot,xguse/ggplot,xguse/ggplot,mizzao/ggplot,smblance/ggplot,andnovar/ggplot,assad2012/ggplot,bitemyapp/ggplot