Hi There
I've written my own graphing software before using just php, but using a third party resource like is simple as is allowing custom data entry.
I've allowed 3 days so we can both be 100% certain you are happy with the final product, but it should be ready for testing/tuning within a day.
My wife and I run a home school, so helping students is a passion, so I hope I can be of service to you.
Many Thanks