how to create a program with a chart

ı am tryıng to fınd an flow variable for any S and l/d ratio that is specified by user. suppose ı said (x axis) S=0.20 and l/d=0.60. how can ı calculate the corresponding y axis value? on the chart only 4 types of l/d ratio is given(inf,1,1/2,1/4). but ı should be able to calculate for any l/d ratio. its one step of my final thesis but ı struggle for a long time and ı still couldnt solve it. ı hope someone can help me. thank you in advance...