To perform this, each of the rolls you want to compare must be saved as a group of rolls, and then assigned to a new roll template. Once that's done, you can use the new roll template (click the icon/button), and you'll be presented with the series of rolls together. (You can also hit the "Roll multiple groups of dice" button from the dice interface at the top of the page.)
For example, to create a group of dice rolls for a specific situation, you'd select which dice rolls you want added to the group by clicking a gray box around them (you'll see your mouse cursor change to the "+" icon), and then click the "Roll all selected rolls" button (in the list to the right of the image).
Note that this functionality requires either that the current roll template you're creating a group for be first in the list, or that the template be populated. You can't add rolls to un-populated roll templates.
When done, you'll be able to see the group of rolls in the list to the left, and be able to select them as "active rolls". Once you select an active roll, you can see the corresponding roll results in the chat box.
In addition, by default when creating a new roll template, you'll see a button to roll up to 10 groups of dice. (If you don't, click the button and you'll automatically be rolled up.) You can hit the "Roll all selected rolls" button for the newly-created roll template (in the list to the right of the image) to roll all of your selected rolls together. Alternatively, you can always hit the "Roll all selected rolls" button from the dice interface at the top of the page.) d2c66b5586