The graph icon in the 'Campaign overview' section of reporting indicates the availability of a graphical representation of your delivery, engagement and interaction levels respectively.
Click on the graph icon to open it up.
Clicking the graph icon against 'Delivery' displays a pie chart showing a visual representation of your delivery rates. This is broken down into emails delivered, hard bounces, soft bounces and unsubscribe requests.
Clicking the graph icon against 'Engagement' displays a line graph, with three different tabs, showing a visual representation of open rates per day, per time of day (broken down into hours) and in the first 24 hours.
You can zoom into these line graphs for greater detail and clarity by clicking, dragging across the desired section and then releasing the mouse button. Click on Reset zoom to return to the original view.
Clicking the graph icon against 'Interaction' displays a bar graph, with three different tabs, showing a visual representation of the volume of clicks for each link in your email per link (URL), per link name and per link group.