Google Spreadsheet Last X Rows

You can create a new sheet and use the QUERY function to get the last 90 rows:

• If column A is unique and sortable, you simply sort your data in reverse order and take the top 90 rows, and then reverse sort it again: =SORT(QUERY(Sheet1!A:Z,"order by A desc limit 90"),1,1)
• If that is not the case, then you need to calculate the offset by finding the last row and subtracting 90 from it, and then query the data to return 90 rows starting from that offset: =QUERY(Sheet1!A:Z,"limit 90 offset "&(COUNT(Sheet1!A:A)-90))

You can then use this sheet to generate your chart.