Need to find curve/trendline and equation that fits the large data set. Looks as in the scatter plot given below. any suggestion in a particular direction are appreciated. Any libraries in java or python or any online tools. I have tried excels trendline option, but the equation underfits my data.
That looks like data you want to transform first and then maybe try a linear model. My first guess would be to try regressing $y$ against $1/x$