The kernel used for KDE. One of "gaussian", "cosine", "optcosine", "rectangular"/"uniform", "triangular", "biweight"/"quartic", "epanechikov"/"parabolic". Only used when method = "kde2d".
"gaussian"
"cosine"
"optcosine"
"rectangular"
"uniform"
"triangular"
"biweight"
"quartic"
"epanechikov"
"parabolic"
method = "kde2d"