Is it OK to extract datasets from somebody's paper and work with it in your own papers? Let's say David Smith has published a paper a while ago. In this paper there are some results plotted in a graph. I want to use this dataset in my own research and extend it with my own results. Unfortunately, it is not possible to contact David Smith. So I use some of tools mentioned for example in this question to extract the dataset from the plot (which, of course, might introduce some kind of small errors).

Do you consider it as OK if I now publish a paper which includes the old dataset from David Smith + my new data in a new plot? I would, of course, cite the old paper by David Smith. (I might deduce some results from the old+new data that a somewhat different that the original conclusion by David Smith.)
If you cite the original authors in your plots and when comparing results/conclusions (or simply everywhere you use their data/work) you should be fine.
Also make it clear that the data is "reverse engineered" from their graphs/plots and not the "original" data.