Plotly is a free online analytics and data visualization tool that you can create online graphing, analytics, and stats tools for individuals and collaboration, as well as scientific graphing libraries for Python etc.
It is extremely easy to create graphs with your own data that you complete on the site or you can import your own data from things like Google drive, Excel or text files. You can then tweak and add stats and functions to this data too.
You can then customize the graphs, annotate them etc. In addition there are lots of tutorials built into the site which help you handle the data if you are not that confident and also other resources which may be useful as part of the Plotly community.
Finally another potential interesting feature is the real time graphing which could be particularly useful for data logging etc.
I have only had a play this morning but it looks a really interesting site. Have you used it? What do you think?
You can follow Plotly on twitter too @plotlygraphs