It took me a while to understand this module. Let's simplify it so that reading the code explains what the code is doing. Some suggestions:

1.   Move the success_qps, error_qps etc data holders to class variables. This way we avoid the overhead of passing them as method args, and I think the code will also be easier to read.
2.  Check line_data['s'] == true in parse_xml_jtl method so that the right method is called (success or error), instead of calling a generic method and determining success or error inside the method.
3.  Similarly, consider combining some of these helper methods. We can have only one helper method for each 'sub-metric' (e.g., 3 methods for qps, response_times, response_sizes respectively)
4.  Document the helper methods.