Plot Magnitude from RBJ Biquad
Equation (18) in the pdf is correct to calculate the magnitude response for biquad filters. 

As you might know the magnitude response is plotted over the frequency. So you will need to define your points on your x-axis, which resembles the frequency $f$. 

In digital systems your frequency will go up to $f_s/2$, where $f_s$ is the sample rate. The $\omega$ is just $2\pi f$. So you just have to subsitute $\omega=2\pi f$ in Eq.(18) and calculate the magnitude response $|H(e^{j 2\pi f})|$ (y-axis) for desired frequency points (x-axis). 

You will end up with two vectors. One is the vector of frequencies $\mathbb{f}$ that you define. And the other vector is your calculated magnitude response $|H(e^{j 2\pi f})|$. 

I have not used WDL/IPlug frameworks, but I guess you can then just plot your graph using the DrawPoints(x,y) function, using the calculated value pairs. You may have to look into the documentation of the frameworks for further hints.
Depending on the resolution of the plot, you may want to increase the step size of your frequency vector, to increase performance.