How do I get the equation for trendline in apps script?

[Taylor King]

I am trying to use apps script to automatically build charts from a google spreadsheet, but also project what may happen in the future using trendlines. I have figured out how to create the chart with a trendline equation in the title, but I need to do a few more steps:

1. Create multiple types of trendlines (linear, exponential, different types of polynomials, logarithmic, etc.)
2. Compare the R^2 values of each trendline and use the one closest to 1 to do step 3.
3. Extract the exact equation of the trendline and use it to project data.
4. Place data in spreadsheet

Here is what I have so far:

function createChart() {

  var trendlinesopt = {

    0: {

      type: 'linear',

      color: 'black',

      lineWidth: 1,

      opacity: 0.2,

      showR2: true,

      visibleInLegend: true,

    }

  };

  var chart2 = formulaSheet.newChart().asScatterChart()

    .addRange(formulaSheet.getRange(2,6,15,2))

    .setPosition(1,1,5,5)

    .setOption("trendlines",trendlinesopt)

    .build();

  formulaSheet.insertChart(chart2);

}

I can generate multiple charts easily, this is just consolidated to the linear function. How do I extract the R^2 value and the equation of the trendline?

[Pat Ng]

For the equation of trendline - here is one answer.

https://stackoverflow.com/questions/45413670/google-charts-get-the-equation-of-linear-trendlines

I would also appreciate reference on getting R^2 .