font = FontProperties()
font.set_name('Microsoft YaHei')
plt.text(0.02, 0.98, "℃", size=18, color='white',
ha="left", va="top", fontproperties = font,
transform=ax.transAxes,
bbox=dict(facecolor='black', alpha=0.4, pad=0.2, boxstyle="round", linewidth=0)
)