You update the fontsize of a matplotlib.text.Text using text.set_fontsize(). You can use a "resize_event" to call a function that sets a new fontsize. In order to do this with every text in a plot, it might be helpful to define a class that stores initial figure height and fontsizes and updates the fontsizes once the figure is resized, scaled by the new figure height divided by the initial one.
You may then also define a minimal readable fontsize, below which the text should not be resized.
A full example:
import matplotlib.pyplot as plt
import numpy as np
class TextResizer():
def __init__(self, texts, fig=None, minimal=4):
if not fig: fig = plt.gcf()
self.fig=fig
self.texts = texts
self.fontsizes = [t.get_fontsize() for t in self.texts]
_, self.windowheight = fig.get_size_inches()*fig.dpi
self.minimal= minimal
def __call__(self, event=None):
scale = event.height / self.windowheight
for i in range(len(self.texts)):
newsize = np.max([int(self.fontsizes[i]*scale), self.minimal])
self.texts[i].set_fontsize(newsize)
fontsize=11
text = plt.text(0.7, 0.6, "Some text", fontsize=fontsize,
bbox={'facecolor':'skyblue', 'alpha':0.5, 'pad':10})
cid = plt.gcf().canvas.mpl_connect("resize_event", TextResizer([text]))
plt.show()