One "Bad" chart does not necessarily mean you  have "failed" the examination.  The charts are  numerically scored and the overall score on each question determines "pass or fail".  You will be asked for an explanation on the one question on one chart in some cases, if the overall score of three charts shows significant responses, you will be asked for an explanation and the test result would indicate deception to that question.  If it is a multiple question test, ""screening examination", you would be administered another examination on a different day, and only the question you were having problems with would be on the examination.  

Does the polygrapher report on just a pass/fail litmus based on charts, confirmed by admissions, or can people who produce a 'bad' chart or a chart with 'problem areas' still pass based on how they react to the polygrapher's interrogation in the post-testing interview?

The post test interview would not alter the overall score on the chart/charts and your charts would be reported as "Significant Response" to that question.  The testing is not finished until the one question showing significant response is resolved with further testing on that issue only.  Not all examiners conduct polygraphs in this manner, however, it is the standard established by the polygraph industry at large, i.e. APA and AAPP.