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)

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