I frequently have to review performance data for customers while doing performance troubleshooting and capacity planning. Kudos to Nigel Griffiths for his excellent NMON tool and associated programs. NMON makes collecting performance data on AIX and Linux a breeze.
Analyzing that data is fairly simple. There are a variety of tools, and I have typically used the Excel based NMON Analyzer . I feel that the graphs are quite good and because it is in Excel it's easy for me to annotate and share with customers. It has the option to merge data files, unfortunately due to Excel's constraints it is limited to about five days of data.