All common waveplate materials exhibit nonlinear dispersion of retardance, producing an unwanted chirp in the interference fringes that channeled spectropolarimeters use for heterodyning polarization data. After showing how to quantify this nonlinearity, we survey the common waveplate materials and find that MgF2 has significantly lower nonlinearity than any other available material. We also quantify the degree of crosstalk caused by dispersion nonlinearity and show that, unlike in linear dispersion, the degree of crosstalk depends on the sequence of how the phase calibration is implemented. Regardless of how the calibrated phases have been obtained, shifting each channel to baseband prior to windowing minimizes crosstalk error.