The top chart represents results of a Public Policy Polling survey -- done between January 8th and 10th of a random sample of 580 likely Democratic voters in Iowa, with a margin of error of 4.1 points.
The second chart reflects the results of a Quinnipiac University Poll -- done between January 5th and 10th of a random sample of 492 likely Democratic voters in Iowa, with a margin of error of 4.4 points.
You may notice that the two polls disagree about who is leading in Iowa. The PPP survey has Clinton with a 6 point lead, while the Quinnipiac poll has Sanders with a 5 point lead -- both slightly above the margins of error.
CNN has been touting the results of the Quinnipiac poll, and ignoring the PPP survey. That's no surprise, since it helps their ratings to portray Clinton as being "in trouble". But the truth is that the PPP survey is at least as reliable and prestigious as Quinnipiac, and the best that can be said is that the candidates may be growing closer in Iowa.
I still expect Clinton to come out on top in Iowa, but both candidates are likely to exit that state with a roughly equal number of delegates. It probably won't be until Super Tuesday (March 1st) that Clinton take a significant lead in delegates.