Rather than grouping by the date you should try to just order by the date. This should give you all of the dates in order. If there is a record that has a date it will be listed and the value for that date plotted on the chart.
As for the sorting options being lost you can check on the data page inside of the DynamicWebCharts folder in your site. You should be able to see the grouping and order in here.
If you have any further trouble with this please post back and include a copy of the db in a sql file and the page and DynamicWebCharts folder. I can check it out for you to see what is going on.