Yellow bars represent sentences that are important and therefore appear in the summary.
Purple bars represent sentences that are less important and were thrown out. If you let your mouse cursor hover over a bar, the sentence that the bar represents appears to the right of the graph. By blessing and hiding sentences, you can alter a summary to suit your tastes. To bless a sentence, hold down the Option key while clicking on it's bar in the graph. To hide a sentence, hold down the Command key while clicking on it.