Just plot each line and then plot another line between the end of line 1 and the beginning of line 2. This will do it.
Ra = (2*P*(1-a)+D+S+T*b)/2;
Rb = (2*P*a+D+S+T*(2-b))/2;
numPoints = 100
T1 = @(x) x*(Ws+Wd)-Ra;
T2 = @(x) x*(Ws+Wd)-Ra+P;
T3 = @(x) Rb-(L-x)*(Wt+Wd+Ws);
x1 = linspace(0, a*L, numPoints);
x2 = linspace(a*L,L-b*L, numPoints);
x3 = linspace(L-b*L,L, numPoints);
y1 = T1(x1);
y2 = T2(x2);
y3 = T3(x3);
plot(x1, y1, '-', 'LineWidth', 3);
hold on;
plot(x2, y2, '-', 'LineWidth', 3);
plot(x3, y3, '-', 'LineWidth', 3);
% Plot black line that jumps the gap between line 1 and line 2.
plot([x1(end), x2(1)], [y1(end), y2(1)], 'k-', 'LineWidth', 3);
ylabel('Tvärkraft [N]');
xlabel('x[m]')
title('T-diagram')
grid on;
