Hello Zaref,
The above code you provided appears to be correct for plotting the graph of channel capacity in SNR(dB) provided the channel is a fixed bandwidth ("B" in the above code) Gaussian Channel in the presence of additive Gaussian noise. This relation is called the Shannon-Hartley Theorem. The graph is also the accurate representation of the same.
SNR_dB = -20:.1:30;
B = 1; % BW (Hz)
C = B * log2(1 + 10.^(SNR_dB/10));
plot(SNR_dB, C, 'LineWidth', 1);
xlabel('SNR (dB)');
ylabel('Channel Capacity (bit/s)');
grid on;
