Error in sci-kit learn version of DifferenceInDifferences.plot method

[drbenvincent]

When running the did_skl.ipynb example we get an error when calling the plot function.

ValueError: setting an array element with a sequence. The requested array has an inhomogeneous shape after 2 dimensions. The detected shape was (2, 1) + inhomogeneous part.

I believe this relates to line 453 here where we calculate the position of the text annotation associated with the causal effect arrow.

CausalPy/causalpy/skl_experiments.py

Lines 451 to 459 in f1a522d

	ax.annotate(
	"causal\nimpact",
	xy=(1.05, np.mean([self.y_pred_counterfactual, self.y_pred_treatment[1]])),
	xycoords="data",
	xytext=(5, 0),
	textcoords="offset points",
	color="green",
	va="center",
	)