Fix: guard against unicode markers (problem in matplotlib 1.3.1)
Fix: guard against unicode markers (problem in matplotlib 1.3.1)

matplotlib 1.3.1 is not yet unicode ready and will not take
unicode markers. So add a `str(..)` to make the not unicode
under py2.

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