By "Diagram" I assume you mean the chemical structure or structural formula of an OCTATRIENE. Draw eight C's in a line. Those are your eight carbon atoms, Now connect any three to the next one with a double line ( = ). Those are your double bonds. Now add single lines ( - ) so that each C has four lines (bonds) total. Put a H atom at the end of each line and you will have your chemical structure. It will have the formula C8H12. You can put the double bonds between any of the carbon atoms you wish, You can also make branched chains (C atoms coming off a longer string of atoms) or you can put the atoms in a ring; all thes will be ISOMERS of your original compound, Try it.

One last thing, make sure that each carbon atom has a total of 4 bonds :)