After enabling feed tracking on dashboards, your users can post snapshots of dashboard charts on the dashboard's feed. Select Enable Dashboard Component Snapshots from the Reports and Dashboards Settings page in Setup to allow users to post charts to group feeds, or to their own Chatter feed. If you don’t enable feed tracking, users can post charts to the dashboard's feed, ensuring that only people with access to the dashboard see the chart. Posting a chart to a person's or group's feed allows someone without access to the dashboard to see the chart.
After sharing the chart, mention people and groups by commenting on your post.