Adjusting Text background transparency
The alpha passed to plt.text() will change the transparency of the text font. To change the background you have to change the alpha using Text.set_bbox():
t = plt.text(0.5, 0.5, 'text', transform=ax.transAxes, fontsize=30)t.set_bbox(dict(facecolor='red', alpha=0.5, edgecolor='red'))#changed first dict arg from "color='red'" to "facecolor='red'" to work on python 3.6