docs: def x_position(self, filter_order=None, window_size=None, tol=0.05,
                     Lx=None):
          '''
          Calculate $x$-position according to:
  
                 __ | C       |
                             ╲╱ a   ⋅ | - - c_f |
                    | a       |
          x = ──────────────
                c_d - c_f
  
          where:
  
           - $C$ is the measured capacitance.
           - $c_f$ is the capacitance of the filler medium per unit area
             _(e.g., air)_.
           - $c_d$ is the capacitance of an electrode completely covered in
             liquid per unit area.
           - $a$ is the area of the actuated electrode(s).
  
          Note that this equation for $x$ assumes a single drop moving across an
          electrode with a length along the x-axis of Lx. If no value is provided
          for Lx, the electrode is assumed to be square, i.e.,
              Lx=Ly=sqrt(area)
          '''
          if self.calibration._c_drop:
              c_drop = self.calibration.c_drop(self.frequency)
          else:
              c_drop = self.capacitance()[-1] / self.area
          if self.calibration._c_filler:
              c_filler = self.calibration.c_filler(self.frequency)
          else:
              c_filler = 0
          if Lx is None:
              Lx = np.sqrt(self.area)
          return (self.capacitance(filter_order=filter_order,
                                   window_size=window_size, tol=tol) / self.area \
                  - c_filler) / (c_drop - c_filler) * Lx