You can set one line in the chart as the baseline by which the others are measured. To do this, click the small graph icon next to the ticker symbol for the stock that you want to use as the baseline (shown in the screenshot below). This “flattens” the line, setting it at 0%, and thus shows the other stocks’ relative performance. The baseline stock will show a flat line in its graph icon, while the others will show squiggly lines in theirs. Any line in the chart can be set as a baseline by clicking on its graph icon—this also means you can switch baselines just by clicking on a different graph icon.
You can also set a baseline by simply clicking one of the lines in the legend. The line that is set as a baseline will be underlined. Click the ticker again to remove the baseline setting.