Free (or readily available) easy to use plotting software


I'm giving an upcoming talk to a mixed group of teachers, school admins, and researchers (mostly qualitative) on data visualization. The topic is the cognitive aspects that make a graphic good.

My question: What are some free (or easily accessible) easy to use software for plotting?

I have: (ggplot in the clouds) (many eyes) (tableau) Good old excel
And here is another top 20 with free, open-source and paid-for data visualization tools:

D3.js is unparalleled for visualizations, but only if you know or are willing to learn to code in JavaScript.

Highcharts is the easiest free tool that requires zero coding. Remember it's only free for non-commercial personal projects.

FusionCharts is only free as a trial, but it's really really worth trying.

For any other questions look at that list. There's a lot there.