Plot a simple line chart with specific range

Dear Friends:

I have several files in a folder. All files have only one worksheet and already have one chart. I wish to add a second line chart with a partial range, that is last 20 rows of column C & column G. Please note that number of rows vary from file to file, i.e, one file may have 400 lines, the other may have 365 and so on! The chart should cover only the last 20 rows