The security market line is very similar to the capital market line with one difference, it is based off of beta. It is the line that CAPM dictates.

def SML(rf,rm,label):
    betas = [x/10 for x in range(21)]
    assetReturns = [rf+(rm-rf)*x for x in betas]
    plt.plot(betas,assetReturns,label=label)
    plt.xlabel("Asset Beta")
    plt.ylabel("Asset Return")
    plt.title("Security Market Line")
    plt.plot(1,rm,"ro")

Notice how the two lines work in a similar manner.

SML(.02,.08,"Line 1")
plt.legend()
plt.show()
SML(.02,.08,"Line 1")
SML(.04,.1,"Line 2")
plt.legend()
plt.show()
SML(.02,.08,"Line 1")
SML(.04,.08,"Line 2")
plt.legend()
plt.show()

Source Code