The Fourier transform of a triangular window function
Not the perfect fit, but a quick-and-dirty base to play on:

%nBin = 81;
nBin = 2049;

y=(20*log(fftshift(abs(fft(triang(81),nBin)))))';
x = linspace(-40,40,length(y));
basey = min(0,min(y));
ShadingColor = [0 0 1];
h = fill([x x(end) x(1)], [y basey basey], ShadingColor); 
set(h, 'EdgeColor','none', 'FaceAlpha', 0.5);