Exactly what is an "elliptical line"? That's a new one on me. I know lines, and I know ellipses, but not an elliptical line. What is it?
Anyway, if you have "some data" on a line, you can get the slope of a line fitted through your "some data" from the polyfit() function.
coefficients = polyfit(x, y, 1);
The slope of the line fitted through the x and y points is