makes logo of DNA given original and mutated DNA sequence

Motif Logo

import matplotlib as mpl
mpl.use('TkAgg')
import matplotlib.pyplot as plt

def graph_logo(ref, alt):

    str = ""

    for num, char in enumerate(ref):
        if char != alt[num]:
            discrep = alt[num]
            d_index = num

    for numb, charac in enumerate(alt):
        if charac != discrep or numb != d_index:
            str += alt[numb].lower()
        elif d_index == numb:
            str += discrep.upper()

    #plt.text(0.5, 0.5, str, fontsize=24)

    plt.text(0.5, 0.5, str, fontsize=24, color='g', weight='bold')

graph_logo("ctt", "ctg")
plt.show()