Plot Spectrum in real time?

I would like to add and plot the spectrum of signal on Qwt Oscillocope's example. My Idea is to create another seriesData class. SerieData. I wonder if there will not conflict between sample of SeriesData and sample of Signaldata or I just need to modify the signaldata? Any help and Advice would be appreciated. Thanks

input signal is usually in form of cyclic buffer the output buffer can be static one. This applies also for time domain plots. Without actually see your structures is impossible to answer if you had to change them or not Here is how I do this:

make some buffer (array) for FFT

can be static ... No changes in it will happen. It size has to be at least slowest_timebase/fsampling samples. In case your FFT has complex domain input then double the size. If you want to scroll/zoom/unzoom then enlarge the size accordingly
find the start sample of the actual Oscilloscope view

via trigger or for starters just last N-samples (but it will flicker because of phase ...) or you preview all samples by N-sample chunks from start to end with the same speed as sampling. Similar like you would send data to sound-card to play sound. You just start after some time so you have enough sampled data already ...
process data

Copy data from start point to FFT buffer add imaginary part of samples if needed (Im=0.0) and process FFT. Then plot the first half of real output.

The frequency of i-th sample (out of N) is f=i*samplerate/N [Hz] where i={ 1,...,(N/2)-1} skipping i=0 which represents DC component.

You can also add logarithmic axises to frequency. In that case do not forget to change the x value in plot too
update start position

simply add to start position the size of used samples (N) and do not forget that trigger also use this time as start point before finding the real start...