I think what you're looking for here is the Connect Points button at the bottom of the Data Inspector. If there is only one series on your graph, you can simply click it with nothing selected after adding the new point. If you have multiple series or the graph is more complicated, you'll want to select the existing series and the new point first before clicking.
Unfortunately the Connect Points feature won't re-style the new point to match the series, but at least everything else will remain correctly styled. I think we can improve on this process in general (as adding a new point to a series is a pretty common operation) and I've added your notes to our development database for reference as we plan future OmniGraphSketcher releases.
I hope this helps! Feel free to post any questions or feedback in this thread, or get in touch via email at
omnigraphsketcher@omnigroup.com.