Excellent first effort

Nice fast app, and the graphs are great! It would help to add a couple of usability things. First, need consistent terms for df across the different distributions. I like the color difference for these.

More importantly, it would be great to add some options after the graph is rendered. Id love to see buttons to allow the user to choose area above, area between two values, and area outside two values (much like the Z table in Rosner). Those options really help my students, and showing them on the figure would be a huge bonus.

Once you update, you should definitely charge for this. I would suggest $1.99 to $4.99.

fendstat about Probability-Distributions