NAs in x and y coordinates

[tmalsburg]

Example:

data(samples)

fixations <- detect.fixations(samples)
head(fixations)

samples$x[100] <- NA
samples$y[100] <- NA
fixations <- detect.fixations(samples)
head(fixations)

What's the proper way to deal with NAs? We can't just delete samples with NAs because the velocity calculations assume that Δt between samples is constant.