Sure, in Latin data is the plural of datum. But in English the word data can be used both as singular or plural (at least according to Webster's New Collegiate Dictionary published by Merriam-Webster).
Quite frankly, it seems to me that the most common usage is rather abstract, i.e., there is no such thing as "a data" (or "a datum" for that matter) in common English usage, nor is there "two data." Rather, there is a data item, two data items, etc. Kind of like water: There is no such thing as "a water" but there is "a glass of water".
And just as we say "water is," we can say "the data is."