Just use matplotlib text function with the "data coordinates" for x and y:

mytext = 'Total: XX \n Mean: YY \n Min Value: AA \n Max Value: BB'
ax.text(x=15000, y='Radial Velocity', 
        s=mytext, style='italic', 
        color='white', bbox = {'facecolor': 'black'}, 
        fontsize=8, verticalalignment='center')
plt.show()