I wanna study about calculating the capacitance value using the graph

Where can I study about it and here is the example graph for this

 

Those tow graphs represent the solutions to a 1st order differential equation. If you have the equation for the solution, you don't need the graph, the values can be readily obtained with a calculator or a spreadsheet.

https://en.wikipedia.org/wiki/RC_circuit

Scroll down to the section on "Time-domain considerations" to see the graphs.