How can I determine a value of gradient from a graph if the line is curved. (not a straight line)

Question

This is where Calculus comes in

If a graph is a curve, then the gradient (slope) of the curve is the slope of the tangent that you draw at a particular point on that curve.

You can do it graphically by drawing the tangent line, but this is not very accurate. If you need to be as accurately as possible, you should read-off many points from the graph in the neighborhood of the point where you want to measure the gradient.

You then fit a polynomial function through the points. You must take the number of points to be much larger than the degree of the polynomial fit function. Then you extract the derivative from the results of the fit.