You need to put the non-local functions at the end, as in the following.
There were several other errors. I don't know if my corrections reflect what you were trying to achieve, but I'll leave you to modify the following as required.
x = 0:0.01:6;
y1 = @(x) zeros(size(x));
y2 = @(x) piecewise1(x);
y3 = @(x) piecewise2(x);
figure; hold on
b1 = plot(x,y1(x),'y'); M1 = "V_G - V_{th} = 0";
b2 = plot(x,y2(x),'g'); M2 = "V_G - V_{th} = 1";
b3 = plot(x,y3(x),'b'); M3 = "V_G - V_{th} = 2";
xlabel('V_D')
ylabel('I_D/K')
legend([b1,b2,b3], [M1,M2,M3], 'Location', 'NorthWest')
function y = piecewise1(x)
for i = 1:numel(x)
if (x(i) > 0 & x(i) <= 1)
y(i) = 1*x(i)-(1/2)*x(i)^2;
else
y(i) = 1/2;
end
end
end
function y = piecewise2(x)
for i = 1:numel(x)
if (x(i) > 0 & x(i) <= 2)
y(i) = 2*x(i)-(1/2)*x(i)^2;
else
y(i) = 2;
end
end
end
