Signal Power estimation
The plot has the power already measured for each sample. If the raw data samples (and not the power results shown here, then the average power can be direclty measured (given by the variance of the waveform). We may think that simply averaging the power measurements in the plot would give us the average signal power--- This would be the average of the squares which is not the same as a true power measurement given by the square of the average. It will be close but such an approach is highly dependent on the statistics of the waveform. For example, if the waveform was a complex Gaussian signal, the average of the squares would under-estimate the true power by -1.05 dB. This is detailed nicely in the app note by Agilent (formerly HP) # 1303 available at this link http://hpmemoryproject.org/an/pdf/an_1303.pdf .
Typically SNR would be determined from the noise power within the occupied bandwidth of the signal while a white noise contribution is spread evenly over the sampling bandwidth. If the signal is oversampled, then you would want to increase the white noise source accordingly to maintain a target spectral density within the bandwidth of the signal.