How to get coordinates of complex RTL-SDR signal?
You are missing several steps.  

You have (complex) time-domain samples.  You need to convert that to scalar frequency domain data to get something like your example plot.  Many steps are required to do that conversion before there are any frequency peaks to be found.

You first need to:  Pick a segment of complex IQ data from the RTL-SDR of length N (you may need to convert the raw RTL-SDR IQ samples from unsigned 8-bit to signed floating point), window it (von Hann or Hamming, etc.), convert it to the frequency domain via an FFT of length N, convert the FFT result to log magnitude, and label the FFT log magnitude result bins by frequency, which will be roughly

frequency(i) = sdr.center_freq + i * sdr.sample_rate / N


for bins 0 to N/2

frequency(i) = sdr.center_freq - (N - i) * sdr.sample_rate / N


for bins N/2 to N-1

Then you can search along that log magnitude array for peaks, and apply the frequency label to the peaks found.