Trend line slop and curve

Hi Im interested to plot XY which is a curve also Im interested to fit a best fit curve through data which should follow the data trend as is and then the point from where curve is changing or having maximum curvature, (X-value at that point) and a best fit line from the trend where values are straight to get intercept, After getting intercept I would like to use intercept and slop to draw another model on same graph. please see the attached figure for detailed idea. Example is shown in figure attached here. Guide me please Thank you

[Star Strider]

No figure. No data.

[Image Analyst]

It was given in his prior post http://www.mathworks.com/matlabcentral/answers/134006-data-fitting-and-slop Jay, why are you repeating the question here? Is there any difference between this and the prior one? Which is the most current?

[Star Strider]

Extrapolating beyond the region-of-fit is not recommended.