FFT analysis using powergui in simulink

I have done 3 set of ode calculation in matlab and got result of 3 waveforms.

I used ode45 for calculation so the time step is not constant.

I got 1850 time samples within the interval 0 to 1sec and 1850 samples of each of 3 outputs.

Now i want to do fft analysis of these 3 waveforms.

For extracting the output data to simulink i generated one more m.file as given below,

I1 = [t y(:, 1)];

I2 = [t y(:, 2)];

I3 = [t y(:, 3)];

[since in workspace t = 1850*1, y = 1850*3]

And used workspace block in simulink like below

i logged the scope data to workspace in scope settings.

After running the model, got below waveform in scope.

I opened FFT analysis option in powergui block. I expected I1 waveform in that block but it is not showing any waveform.

What is reason behind this?