Thank you but I know how to plot graphs. My question is about the way the author calculated the numerical data not ho he plotted it. The question shows the word estimate because I need to estimate the values.
You have to use the fuel-cell base equations (Nernst Dynamic) to calculate either the generating voltage or drops (Ohmic, Const., ...). There are a lot of constants in the mentioned equations which should be set based on your system's dynamic. The current of cells in series is variable due to the power demand (P=V*I).
Use this ref: Ural, Z., Gencoglu, M. T., & Gumus, B. (2007, July). Dynamic simulation of a PEM fuel cell system. In Proceedings 2nd international hydrogen energy congress and exhibition IHEC.