code: def get_saltbridge_frequency(self,analysis_cutoff):
          
          self.frequency = defaultdict(int)
          for traj in self.saltbridges_by_type:
              for contact in self.saltbridges_by_type[traj]:
                  self.frequency[contact["ligand_atom_id"],contact["ligand_atom_name"],contact["resid"],contact["resname"],contact["segid"]]+=contact["frequency"]
          draw_frequency = {i:self.frequency[i] for i in self.frequency if self.frequency[i]>(int(len(self.trajectory))*analysis_cutoff)}
  
          self.saltbridges_for_drawing = {}
          for contact in draw_frequency:
              self.saltbridges_for_drawing[contact]=draw_frequency[contact]
docs: Calculates the frequency of salt bridges throughout simulations. If the frequency exceeds the
          analysis cutoff, this interaction will be taken for further consideration.
          Takes:
              * analysis_cutoff * - fraction of simulation time a feature has to be present for to be plotted
          Output:
              * self.saltbridge_frequency * - frequency of each salt bridge