R mice doesn't give a 'valid' sollution
After a lucky google search, I found the function na.interpolation from the imputeTS-package. After changing the dataset to a time series (XTS), I was able to impute the right data.

library(xts)
library(imputeTS)

set.seed(1337)
df &lt;- data.frame(x = c(seq(100, 10, -4), seq(100, 20, -3), seq(100, 60, -3), 
                       seq(80, 40, -3), seq(100, 10, -3))) %&gt;% data.table()
df[sample(seq_len(nrow(df)), size = nrow(df) * 0.3), x := NA]
df$y &lt;- seq(1,nrow(df),1)

ts &lt;- xts(df[, -c("y")], as.Date(df$y))
head(ts)
#&gt;              x
#&gt; 1970-01-02 100
#&gt; 1970-01-03  96
#&gt; 1970-01-04  NA
#&gt; 1970-01-05  88
#&gt; 1970-01-06  NA
#&gt; 1970-01-07  80

full &lt;- na.interpolation(ts)
plot(full, type = "p")