It is a late answer, but for those, who is searching, and not able to write a python script, I can advice following: 1) from the NCBI genome (or any other source) download proteome file (all in fasta format); 2) download UGENE program, 3) open file in UGENE, choose option "merge sequence in 1", "set X to 0" when merging. 4) On the right bottom panel there will be histogram label - clicking on it you get statistics of all aminoacid in proteome in absolute and percentage values as table, copy-pastable to Excel. Good luck!