Adobe Systems Inc., the world’s biggest maker of graphic-design software, agreed to buy Omniture Inc. for $1.8 billion, expanding into programs that track the performance of Web sites and online advertising campaigns.

Adobe will pay $21.50 a share for Omniture, 24 percent more than the closing price yesterday. Adobe fell the most in almost three months in Nasdaq trading today, a day after announcing the deal and forecasting sales that missed some analysts’ estimates.