How to plot thermal noise in the time domain?
Assume you have a noise signal which you are sampling at sampling rate \$f_s\$. Then, your noise power becomes:
$$v_n^2 = 2kTRf_s$$
Here, \$2kTR\$ is the two sided Power spectral density (PSD).
With this background, you can implement the time domain thermal noise as follows (in MATLAB):
k = physconst('Boltzmann');
T = 290; % Room Temperature
R = 50; % Resistor Value 50 Ohms
fs = 1e6; % Sampling rate of 1MHz.
vn = sqrt(2*k*T*R*fs);
t = (1:1e7)/fs;
th_noise = vn*randn(1,1e7); % Time Domain Noise Signal
P_th = 10*log10(4*k*T*R);
figure(1);plot(t,th_noise);grid on;
xlabel('t (s)');ylabel('Noise Voltage (V)');
title(['Noise Signal with PSD ', num2str(P_th), 'dBc/Hz'], 'Interpreter', 'latex')
figure(2);
pwelch(th_noise,blackmanharris(length(th_noise)/8),[],[],fs,'onesided');

The output is as shown below, first one is the noise signal in time domain, second one is its PSD: