The main point of a commit graph is giving you a visualization to make sense if your git history:
To view the git graph, go to the commits page.
The Show All button
Bitbucket introduced its commit graph since version 6.1 and requested to hide the legacy mode commit by default. The Show All button is not displayed when Bitbucket 6.1's commit graph is enabled. To view the legacy mode commit and display Show All button option, you can find instructions here.
What the show all button does is it shows all branches (even divergent ones), even if they don't return to master. When you select a branch normally, it only shows branches which merge back into the selected branch.