I'm sorry i think i misunderstood your question, as you seem to want to chart an indexed log file and not the index itself.
You could create a simple shell script that will output time, logfile name, and size, (script input) and index where you want, and then generates charts as you need
You can use the following request to get the current index size (size in MB):
| eventcount summarize=false report_size=true index=* | eval size_MB = round(size_bytes/1024/1024,2) | fields index, size_MB
At this point, you can schedule a savedsearch that will populates a summary index including a _time data extracted from the search time itself:
| eventcount summarize=false report_size=true index=* | addinfo | eval _time=info_search_time | eval size_MB = round(size_bytes/1024/1024,2) | fields _time,index,size_MB
And then finally generates chart based on this summary index.