The key thing to remember is that since nufft takes the sample points and frequency points as arguments, you have to remember how to manage their units and how they are defined relative to one another.
The formula for the classical DFT is
and typically we assume that each sample is defined as
where Ts is your sampling period and x() is your analog signal (note that I'm assuming MATLAB's 1-based indexing here, hence the -1 factors). Each represented DFT frequency corresponds to the sampled analog frequency of the spectrum so that X[k] corresponds to the spectral content at an analog frequency
.The generalized NUFFT equation is
In a call to nufft where you are not providing frequency points, the assumed points take the form of
Therefore, if you want results on a comparable scale, you have to scale your sample data to range from 0:(N-1):
fs=1e3; %sampling frequency for the uniform samples
ts=1/fs; %sampling interval for the uniform samples
f0 = 200; %signal frequency under test to be observed at the spectrum output.
timeDataUnif=[0:ts:1000*ts]; %uniform time samples
timeDataNunif=[0:ts:750*ts (750*ts+ts/2):ts/2:1000*ts]; % nonuniform samples for the same time period with twice the sampling frequency
nData1=length(timeDataUnif);
nData2=length(timeDataNunif);
sigDataUnif=sin(2.*pi.*f0.*timeDataUnif) +0.01.*rand(1,nData1);
sigDataUnif=sigDataUnif-sum(sigDataUnif)/nData1; %optionally removing the DC component
fSigData=fftshift(fft(sigDataUnif)); %spectrum of the uniform samples
freqDataUnif=[0:nData1-1]./(nData1-1).*fs-fs/2; %uniform frequency samples
freqDataNunif=[0:nData2-1]./(nData2-1).*(2.*fs)-fs; %uniform frequency grids for twice fs.
sigDataNunif=2*sin(2.*pi.*f0.*timeDataNunif) +0.01.*rand(1,nData2);
% ------ Divide by ts here to set the correct scaling.
nufSigData=fftshift(nufft(sigDataNunif,timeDataNunif/ts));
% ------
figure;
subplot(411),plot(timeDataUnif,'k.');hold on;
subplot(411),plot(timeDataNunif,'r.');title('uniform and nonuniform samples of 1 second time interval');xlabel('sample number');axis tight;grid on;grid minor;
subplot(412),plot(timeDataUnif,sigDataUnif,'k');hold on;title('different samples of the same function');
subplot(413),plot(freqDataUnif,abs(fSigData),'k');hold on;axis tight;V=axis;axis([V(1) V(2) -20 V(4)]); grid on;grid minor;
subplot(412),plot(timeDataNunif,sigDataNunif,'r--');hold on;title('FFT of the uniform samples');
subplot(414),plot(freqDataNunif,abs(nufSigData),'k');title('NUFFT of the nonuniform samples');grid on;grid minor;
