With DSS 1.x (I will update later my post with DSS 2.0 if there is any change), when you explore a dataset or make a preparation script, you work on a sample. As jrouquie suggested, you can change the sample size.
There is something that could help you: the Visualize tab. The normal behavior is that it works on the same sample that with the Explore tab.
But, if you are on a SQL dataset or Impala, you can change the engine and get graphs on full dataset. Read more here: http://doc.dataiku.com/dss/1.4/visualization/sampling.html#live-in-database-engine
I hope that helps.