@Carl Love  Here is the graph. I want to find the function of one of them- for example Aluminium. I can acquire the points from graph by for example xyExtract software, but it causes some inaccuracy.

@Carl Love I thought there must be a more accurate way to do it. thank you very much.

@Carl Love 

I'm not using points. I have a graph and want to know it's function.

@Preben Alsholm 

I finally solved it analytically by HPM method.

@Preben Alsholm 

Merci. Thank you so much.

@Preben Alsholm 

Thanks. You helped a lot.

@Mac Dude 

Thank you very much.