There are quite a few options here, with varying levels of user-friendliness. Some I have used are:
- Grace: Quite simple to use, you input values through a *.dat file, and can manipulate plots through a GUI. I think it's Unix OS only (Linux, MacOS). Speculation: The figures in the question look like they were made in Grace.
- gnuplot: Works on Windows as well as Linux and Mac. A little bit more of a step up in difficulty, as you are manipulating figures either through a script or in a terminal/command prompt.
- Matplotlib: Also works on Windows, Linux and (probably) Mac. Requires some knowledge of Python, since it is a Python package. Also generates figures through a script or IPython console. Found this to be a bit easier than gnuplot. Very good documentation.