When displaying summary_plot, the color bar does not show.
shap.summary_plot(shap_values, X_train)

I have tried changing plot_size. When the plot is higher the color bar appears, but it is very small - doesn't look like it should.
shap.summary_plot(shap_values, X_train, plot_size=0.7)

Here is an example of a proper looking color bar.

Does anyone know if this can be fixed somehow?
How to reproduce:
import pandas as pd
import shap
import sklearn
from sklearn.ensemble import RandomForestRegressor
# a classic housing price dataset
X,y = shap.datasets.boston()
# a simple linear model
model = RandomForestRegressor(max_depth=6, random_state=0, n_estimators=10)
model.fit(X, y)
shap_values = shap.TreeExplainer(model).shap_values(X)
shap.summary_plot(shap_values, X)
In this case, the color bar is displayed, but it is very small. I have chosen such an example to make it easy to retrieve the data.
I had the same problem as you did, and I found that the solution was to downgrade matplotlib to 3.4.3.. It appears SHAP isn't optimized for matplotlib 3.5.1 yet.
I had a similar issue and I was using max_display=10. The below code solved my problem:
plt.gcf().axes[-1].set_aspect(100)
plt.gcf().axes[-1].set_box_aspect(100)
As mentioned above, it seems that the handling of colorbar or box aspect ratio has been changed in matplotlib.pyplot version 3.5. However, you can correct that.
shap.summary_plot(..., show=False) to allow altering the plotplt.gcf().axes[-1].set_aspect(1000)plt.gcf().axes[-1].set_box_aspect(1000)This gives you the old result back. If you want to make the colorbar thicker, set the aspect to 100.