Draw a line, output equation? 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?
 A: I think what you're looking for is curve fitting. https://mycurvefit.com/ is one tool that can do it.
A: Another suggestion is https://www.geogebra.org/graphing which has several other functions and also lets you draw point by point with the cursor. It takes some time to learn the mini programming language that it provides but if you learn it it can be very powerful. Check the "Best Fit Line" tool or the Spline(...) command for parametric curves. It's hard though to get the values out, so I ended using the Browser inspector tool to look at the HTML of the site.
A: 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. 
