Compare Performance Baseline

The historic performance data dashboard lets you load 2 datasets - the current and also a second comparison
Load the comparison data after loading your initial historic data by clicking the "Compare" dropdown and select how long prior to the current loaded data you want to see. If you have loaded 1 hour of data the comparison will be 1 hour long. For example if you have 1 hour of data loaded for Friday and load a comparison 1 day prior - the same hour on thursday will be loaded.
SQL Server history viewer button

Then performance charts will show a dotted line for the comparison baseline data and the currently loaded data will continue to be shown as before.
SQL Server history viewer selection
Alternatively you can load whole days of comparin baseline data from the instance/date control on the left of the screen and select either "Load History" or "Load as Comparison" buttons:
SQL Server history viewer selection
The first button loads the data normally the second loads as the dotted comparison data lines.

Compare SQL Performance Baselines

When the 2 datasets are loaded you will be able to start comparing the orignal set (the baseline) with the new comparison data.
Unexpected anomolies and trends should stand out visually and allow you to home in on problem areas. Increased comsumption of memory and changing transactional throughput over time are particularly easy to identify using the comparison baselines.
It is always good to occassionally compare a current day of performance to a day from a few weeks prior to see if there are any negative performance traends that could become a problem. This preventative trend analysis can be performed using t-sql scripts and spreadsheets plus lots of your time without miniDBA but using the comparison baseline makes it a low difficulty task.