It is a superposition of shifted sinc() functions.
The bold curve (with peak at 6) shows clearly one of the sincs.
It can be:
1) an example of representation in time domain of an ideal binary NRZ unipolar transmission with minimum bandwidth: each sinc is the response a pulse (pulse="1", nopulse="0"). In this case, x-axis is time in units of Tbit.
2) a representation in frequency domain of OFDM. Each curve is a spectrum corresponding to a subcarrier (subcarriers 5 and 7 are not present). In this case, x-axis is frequency in units of 1/Tsymbol.
From the context, it seems it is 2)
Regards
Z