What is the formula for induced drag? I am looking to create one of these graphs for a simple model I tested in a wind tunnel. 


I have managed to define the lift coefficient and thus I've been able to create the parasitic drag line. The problem is that I am having issues with finding the correct formula to use for the induced drag. I have been looking a while now and the only formula's I have found had the induced drag increase with a higher velocity.

the problem i'm running into is that the formula i see around is 

$D_i = \frac{L^2}{\frac{1}{2}\rho V^2   \pi b^2 \epsilon}$ 

this formula has Lift squared above the divider and lift itself is dependant on velocity so whenever I use that formula the graph I get is of one that increases with speed instead of decreases.

What I'm asking for is the correct formula to use here
It seem your graph of induced lift is not decreasing because you assume that the lift increases with velocity. This is generally not the case. 

Typically, a drag vs velocity graph is made for unaccelerated level flight. Under these conditions the lift is equal to the weight of the aircraft. 

$L = W =  \frac{1}{2}\rho V^2 c_L S$

From this we can obtain the lift coefficient as a function of velocity:

$c_L =  \frac{W }{\frac{1}{2} \rho V^2 S}$

The drag of the  aircraft is the sum of the parasite drag and the induced drag:

$D = D_p + D_i$

With the parasite drag: 

$D_p = c_{D,0}\frac{1}{2} \rho V^2 S $

And the induced drag:

$D_i = \frac{1}{2}\rho V^2 S \frac{c_L^2}{\pi  AR \epsilon} =
\frac{W^2}{\frac{1}{2}\rho V^2 S \pi AR \epsilon}  = \frac{W^2}{\frac{1}{2}\rho V^2   \pi b^2 \epsilon}$ 

It is important to understand that this only holds when the lift is equal to the weight of the aircraft (e.g. straight &amp; level flight)

Nomenclature:
    $L\:\:\:\:\:$ lift
    $W\:\:\:$ aircraft's weight
    $\rho\:\:\:\:\:$ air density
    $V\:\:\:\:$ velocity
    $S\:\:\:\:$ wing surface area
    $c_L \:\:\:$ lift coefficient
    $c_{D0} \:$  zero-lift drag coefficient
    $\pi \:\:\:\:\:$  3.14159$\dots$
    $AR \:\:$  aspect ratio of the wing
    $\epsilon \:\:\:\:\:\:$  the wing's Oswald factor
    $b \:\:\:\:\:\:$  wing span