It can depend on the type of graph you have made. If you make a scatterplot you should see that option. If you are graphing categorical attributes or you are making distribution graphs you won’t. Feel free to create a share link to your doc if you want more specific feedback on the graph you are trying to add connecting lines to.
You can click on the y-axis and choose “Treat as numerical” which will create a scatterplot and you’ll have the connecting line option.
You can click on the attribute name in the table (the table header for the Deer Population column) and then select “Edit attribute properties“, after which you can set the attribute “Type” to be “numeric.” Once you do that all future graphs will treat this attribute as numeric, so you can remake the graph.