The size grid is logarithmic, so the interpolation is too. A series of a single point can only speak for the coordinate it sits at, and a coordinate that is not positive has no logarithm, so those two cases fall back to matching the coordinates exactly and to a linear interpolation respectively.
interpolate_in_log_size(x, y, xout)The interpolated values, NA where xout is outside the range of
x.
The coordinates and values of the series.
The coordinates to interpolate onto.