Hey Ivan! Julianne from Mixpanel Support here.
I hate to be the bearer of bad news, but it isn't possible to save the Retention report chart to a board. That would be a product gap on our end.
I definitely recommend submitting your use case and request for this feature via the 'Submit Feedback' form (under the ? menu). This will flag your use case and support of the feature to the Product team, which they use to develop future releases and improvements
