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The recommended method to indicate the data type of a matplotlib Figure object in Python3 using type hints is:

import matplotlib.figure as fig
from typing import TypeVar

Figure = TypeVar('fig.Figure')

Then, you can use Figure as the data type annotation for any variable or function parameter that expects a Matplotlib Figure object. For example, if you have a function that takes a Matplotlib Figure object and a NumPy array as inputs and returns a Matplotlib Figure object as output, you could write:

import numpy as np

def plot_data(fig: Figure, data: np.ndarray) -> Figure:
    # code to plot data on the figure
    return fig

This indicates that the fig parameter should be a Matplotlib Figure object, the data parameter should be a NumPy array, and the function should return a Matplotlib Figure object.