Step artefact in ROC curve with sparse scores

[HuwCampbell]

Classifier plots roc curve drawing appears to have visual artefacts when scores aren't spread out well (there are a large number of instances with the same score, leading to empty deciles).

[HuwCampbell]

related http://blog.revolutionanalytics.com/2016/08/roc-curves-in-two-lines-of-code.html