The graph displays the progress you've made in your cycle.
Cycle graphs autogenerate once a cycle has begun and update every hour.
When viewing a specific cycle, open the cycle sidebar with Cmd/Ctrl I to view the graph.
From the "All Cycles" view, you'll also see the graph belonging to the current cycle.
We have separate lines for Started and Completed issues. This lets you see exactly what is being worked on and what is done.
The blue dotted line represents the "target"—an even distribution of the total estimated points within the cycle over the remaining days in the cycle. Note that it flattens over weekends due to the expectation people do not work during weekend. If your progress stays above that line, then you will complete everything within the cycle.
You can see how much scope (by estimate points) has been added or removed since the start of the cycle. Hover over the graph to see the exact values of scoped, completed and started issues over time.