Draw a line, output equation?

Question

I am looking for an app or computer program that can spit out an equation of a line that you draw. I don't know any programs like this, so maybe someone can help?

Answer 1

http://www.padowan.dk/download/ I use this graph calculator.
It has a feature:

Point series and trendlines
You can create series of points with different markers, colors and size. Data for a point series can be imported from other programs, e.g. Microsoft Excel. It is possible to create a line of best fit from the data in a point series, either from one of the built-in models or from a user-specified model.

which clearly does what you want.

Answer 2

I think what you're looking for is curve fitting. https://mycurvefit.com/ is one tool that can do it.