Pelle's C aims to support C99, so everything that is defined by that standard is available and in fact is described in the online help. Just starting the "Contents" of the online help give you a table of all intrinsic functions in a table, there isn't much else you could possibly need.
And in the "Contents" tab of the help, yes, you can "view" all the include files (.h) just fine, with an example for each and every function. Not sure what more you are looking for in this case.
As far as programming for Windows goes, the best way to get the Microsoft Windows SDK and associated documentation.
And as far as "producing graphs" goes, Pelle's C does not include any special graphics libraries, but there is an abundance of 3rd party libraries around, like the SDL stuff. For this and possibly others, examples should be in the "User contributions" and "Graphics programming" sections of this this forum...
Ralf