Delta aggregation of incremental data?

I'm collecting data about disk space usage, basically it's result of df /|awk 'NR==2 { print $4 }' (free space in bytes), so the value is decreasing through time in this example, is there any way to process those data so i have some statistic about how given numerical field changine over time, so i can visualize changes for example in kibana or even query value's net change in given time