The percentage of undergrad women CS majors didn't change significantly between 2015 and 2016.
It appears to be slightly lower (26% vs. 30%) on the 2016 chart because that chart includes grad students.
However, even if the percentage of undergrad majors had dropped from 2015 to 2016, that by itself wouldn't tell us anything about the retention or graduation rate over time, since we don't know what the breakdown was for 2015 graduates, CS majors declared or dropped in 2016, etc..
https://www.stanforddaily.com/2016/11/08/graphic-gender-rati...
It would seem declared majors aren't following through to the end of their programs.