Shiver. You have data that lies between 14.5 and 15.9, and you wish to extrapolate down to x = 9???????????? Have you learned too little from the works of Mark Twain?
“In the space of one hundred and seventy six years the Lower Mississippi has shortened itself two hundred and forty-two miles. That is an average of a trifle over a mile and a third per year. Therefore, any calm person, who is not blind or idiotic, can see that in the Old Oölitic Silurian Period, just a million years ago next November, the Lower Mississippi was upwards of one million three hundred thousand miles long, and stuck out over the Gulf of Mexico like a fishing-pole. And by the same token any person can see that seven hundred and forty-two years from now the Lower Mississippi will be only a mile and three-quarters long, and Cairo [Illinois] and New Orleans will have joined their streets together and be plodding comfortably along under a single mayor and a mutual board of aldermen. There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” "Life on the Mississippi", Mark Twain, 1884
Ok. if you insist on doing this extrapolation down to an arbitrarily virtually random value at x = 9, then use a tool that can do it in style, and will have the properties you desire for that curve. My SLM tools will do exactly what you need.
slm = slmengine(x,y,'plot','on','knots', ...
[9 14.5 15 15.5 16],'increasing','on','concaveup','on');
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