No, N is not a vector it's a number, though you can make a vector from all the Ns:
allN = [2, 4, 8, 16, 32];
plot(ecg, 'b-', 'LineWidth', 3);
grid on;
for k = 1 : length(allN)
% Get this one N
N = allN(k);
% Construct kernel
h = ones(1, N) / N;
% Do the convolution, cropping to the same size as the input vector.
output = conv(ecg, h, 'same');
% Plot it.
hold on;
plot(output, '-');
% Create the string for the legend since all plots show up in different colors.
legendStrings{k} = sprintf('N = %2.2d', N);
end
legend(legendStrings);
