Prevent the tooltip or popover from being cut off at the edge of the graph.
Specific tooltip content with links.
Highlight values in a series with a different color.
Add a unit on Y axis and in the tooltip.
You may want to specify the color of a series in the series itself, regardless of its index.
The chart may need to be redrawn if it is placed in a div that changes size.
You may want to position legends vertically or to separate them into different containers.
You may want to add a timeline slider under the graph.