I am not sure what do you mean by combining the graphs? In my case I just generated the curves one by one and then copied/pasted them into a single graph.
The reason for the different length in the case of the LDPC code is due to the dimensions of the used parity check matrix. Naturally, the length of the output codeword needs to be a multiple of the parity check matrix dimensions. Since we did not want to apply puncturing, we chose the length multiplier that was the closest to the length of the Turbo code (for fair comparison). By the way, in that paper, the LDPC code used was based on the WiMAX standard. In the current version of the simulator (v1.1), we are using the 5G-NR codes.