BER vs Eb/N0 (db) Curve
Usually, such figures show the error rate on a log scale, and $E_b/N_0$ in dB. So you would specify $E_b/N_0$ in dB, then convert it to a ratio, and use that value to evaluate the formula for the bit error rate (as you did). Then you plot the logarithm of the error rate as a function of $E_b/N_0$ in dB. So the main problem is that you used a linear scale for the BER instead of a logarithmic scale.

If you do things right, you'll end up with something like this: